diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
index 18c0c5a3f8e..553eb4d1477 100644
--- a/.github/CODEOWNERS
+++ b/.github/CODEOWNERS
@@ -108,6 +108,7 @@
 /packages/permission-log-controller             @MetaMask/wallet-integrations @MetaMask/core-platform
 /packages/remote-feature-flag-controller        @MetaMask/extension-platform @MetaMask/mobile-platform @MetaMask/core-platform
 /packages/storage-service                       @MetaMask/extension-platform @MetaMask/mobile-platform @MetaMask/core-platform
+/packages/client-controller                     @MetaMask/core-platform @MetaMask/extension-platform @MetaMask/mobile-platform
 
 ## Package Release related
 /packages/account-tree-controller/package.json  @MetaMask/accounts-engineers @MetaMask/core-platform
@@ -203,4 +204,6 @@
 /packages/claims-controller/package.json @MetaMask/web3auth @MetaMask/core-platform
 /packages/claims-controller/CHANGELOG.md @MetaMask/web3auth @MetaMask/core-platform
 /packages/ai-controllers/package.json @MetaMask/social-ai @MetaMask/core-platform
-/packages/ai-controllers/CHANGELOG.md @MetaMask/social-ai @MetaMask/core-platform
\ No newline at end of file
+/packages/ai-controllers/CHANGELOG.md @MetaMask/social-ai @MetaMask/core-platform
+/packages/client-controller/package.json @MetaMask/core-platform @MetaMask/extension-platform @MetaMask/mobile-platform
+/packages/client-controller/CHANGELOG.md @MetaMask/core-platform @MetaMask/extension-platform @MetaMask/mobile-platform
\ No newline at end of file
diff --git a/README.md b/README.md
index 1efed86de9d..f091822eac7 100644
--- a/README.md
+++ b/README.md
@@ -88,6 +88,7 @@ Each package in this repository has its own README where you can find installati
 - [`@metamask/subscription-controller`](packages/subscription-controller)
 - [`@metamask/transaction-controller`](packages/transaction-controller)
 - [`@metamask/transaction-pay-controller`](packages/transaction-pay-controller)
+- [`@metamask/ui-state-controller`](packages/ui-state-controller)
 - [`@metamask/user-operation-controller`](packages/user-operation-controller)
 
 <!-- end package list -->
@@ -166,6 +167,7 @@ linkStyle default opacity:0.5
   subscription_controller(["@metamask/subscription-controller"]);
   transaction_controller(["@metamask/transaction-controller"]);
   transaction_pay_controller(["@metamask/transaction-pay-controller"]);
+  ui_state_controller(["@metamask/ui-state-controller"]);
   user_operation_controller(["@metamask/user-operation-controller"]);
   account_tree_controller --> accounts_controller;
   account_tree_controller --> base_controller;
@@ -181,16 +183,29 @@ linkStyle default opacity:0.5
   address_book_controller --> base_controller;
   address_book_controller --> controller_utils;
   address_book_controller --> messenger;
+  ai_controllers --> base_controller;
+  ai_controllers --> messenger;
   analytics_controller --> base_controller;
   analytics_controller --> messenger;
+  analytics_data_regulation_controller --> base_controller;
+  analytics_data_regulation_controller --> controller_utils;
+  analytics_data_regulation_controller --> messenger;
   announcement_controller --> base_controller;
   announcement_controller --> messenger;
   app_metadata_controller --> base_controller;
   app_metadata_controller --> messenger;
   approval_controller --> base_controller;
   approval_controller --> messenger;
+  assets_controller --> account_tree_controller;
   assets_controller --> base_controller;
+  assets_controller --> controller_utils;
+  assets_controller --> core_backend;
+  assets_controller --> keyring_controller;
   assets_controller --> messenger;
+  assets_controller --> network_controller;
+  assets_controller --> network_enablement_controller;
+  assets_controller --> permission_controller;
+  assets_controller --> polling_controller;
   assets_controllers --> account_tree_controller;
   assets_controllers --> accounts_controller;
   assets_controllers --> approval_controller;
@@ -201,11 +216,13 @@ linkStyle default opacity:0.5
   assets_controllers --> messenger;
   assets_controllers --> multichain_account_service;
   assets_controllers --> network_controller;
+  assets_controllers --> network_enablement_controller;
   assets_controllers --> permission_controller;
   assets_controllers --> phishing_controller;
   assets_controllers --> polling_controller;
   assets_controllers --> preferences_controller;
   assets_controllers --> profile_sync_controller;
+  assets_controllers --> storage_service;
   assets_controllers --> transaction_controller;
   base_controller --> messenger;
   base_controller --> json_rpc_engine;
@@ -412,12 +429,11 @@ linkStyle default opacity:0.5
   subscription_controller --> polling_controller;
   subscription_controller --> profile_sync_controller;
   subscription_controller --> transaction_controller;
-  token_search_discovery_controller --> base_controller;
-  token_search_discovery_controller --> messenger;
   transaction_controller --> accounts_controller;
   transaction_controller --> approval_controller;
   transaction_controller --> base_controller;
   transaction_controller --> controller_utils;
+  transaction_controller --> core_backend;
   transaction_controller --> gas_fee_controller;
   transaction_controller --> messenger;
   transaction_controller --> network_controller;
@@ -434,6 +450,8 @@ linkStyle default opacity:0.5
   transaction_pay_controller --> network_controller;
   transaction_pay_controller --> remote_feature_flag_controller;
   transaction_pay_controller --> transaction_controller;
+  ui_state_controller --> base_controller;
+  ui_state_controller --> messenger;
   user_operation_controller --> approval_controller;
   user_operation_controller --> base_controller;
   user_operation_controller --> controller_utils;
diff --git a/packages/client-controller/CHANGELOG.md b/packages/client-controller/CHANGELOG.md
new file mode 100644
index 00000000000..acc8b869458
--- /dev/null
+++ b/packages/client-controller/CHANGELOG.md
@@ -0,0 +1,20 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+- Initial release of `@metamask/client-controller` ([#7808](https://github.com/MetaMask/core/pull/7808))
+  - `ClientController` for managing client (UI) open/closed state
+  - `ClientController:setUiOpen` messenger action for platform code to call
+  - `ClientController:stateChange` event for controllers to subscribe to lifecycle changes
+  - `isUiOpen` state property (not persisted - always starts as `false`)
+  - `clientControllerSelectors.selectIsUiOpen` selector for derived state access
+  - Full TypeScript support with exported types
+
+[Unreleased]: https://github.com/MetaMask/core/
diff --git a/packages/client-controller/LICENSE b/packages/client-controller/LICENSE
new file mode 100644
index 00000000000..fe29e78e0fe
--- /dev/null
+++ b/packages/client-controller/LICENSE
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 MetaMask
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/packages/client-controller/README.md b/packages/client-controller/README.md
new file mode 100644
index 00000000000..a9912c2a1c6
--- /dev/null
+++ b/packages/client-controller/README.md
@@ -0,0 +1,180 @@
+# `@metamask/client-controller`
+
+Client-level state for MetaMask (e.g. whether a UI window is open). Provides a centralized way for controllers to respond to application lifecycle changes.
+
+## Installation
+
+```bash
+yarn add @metamask/client-controller
+```
+
+or
+
+```bash
+npm install @metamask/client-controller
+```
+
+## Usage
+
+### Basic Setup
+
+```typescript
+import { Messenger } from '@metamask/messenger';
+import {
+  ClientController,
+  ClientControllerActions,
+  ClientControllerEvents,
+} from '@metamask/client-controller';
+
+const rootMessenger = new Messenger<
+  'Root',
+  ClientControllerActions,
+  ClientControllerEvents
+>({ namespace: 'Root' });
+
+const controllerMessenger = new Messenger({
+  namespace: 'ClientController',
+  parent: rootMessenger,
+});
+
+const clientController = new ClientController({
+  messenger: controllerMessenger,
+});
+```
+
+### Platform Integration
+
+Platform code calls `ClientController:setUiOpen` when the UI is opened or
+closed:
+
+```text
+onUiOpened() {
+  controllerMessenger.call('ClientController:setUiOpen', true);
+}
+
+onUiClosed() {
+  controllerMessenger.call('ClientController:setUiOpen', false);
+}
+```
+
+### Consumer controller and using with other lifecycle state (e.g. Keyring unlock/lock)
+
+Use `ClientController:stateChange` only for behavior that **must** run when the
+UI is open or closed (e.g., pausing/resuming a critical background task). **Use
+the selector** when subscribing so the handler receives a single derived value
+(e.g. `isUiOpen`), and **prefer pause/resume** over stop/start for polling.
+
+UI open/close alone is usually not enough to decide when to start or stop work.
+Combine `ClientController:stateChange` with other lifecycle events, such as
+**KeyringController:unlock** / **KeyringController:lock** (or any controller that
+expresses "ready for background work"). Only start subscriptions, polling, or
+network requests when **both** the UI is open and the keyring (or equivalent) is
+unlocked; stop or pause when the UI closes **or** the keyring locks.
+
+#### Important: Usage guidelines and warnings
+
+**Do not subscribe to updates for all kinds of data as soon as the client
+opens.** When MetaMask opens, the current screen may not need every type of
+data. Starting subscriptions, polling, or network requests for everything when
+`isUiOpen` becomes true can lead to unnecessary network traffic and battery
+use, requests before onboarding is complete (a recurring source of issues), and
+poor performance as more features are added.
+
+**Use this controller responsibly:**
+
+- Start only the subscriptions, polling, or requests that are **needed for the
+  current screen or flow**
+- Do **not** start network-dependent or heavy behavior solely because
+  `ClientController:stateChange` reported `isUiOpen: true`
+- Consider **deferring** non-critical updates until the user has completed
+  onboarding or reached a screen that needs that data
+- Prefer starting and stopping per feature or per screen (e.g., when a
+  component mounts that needs the data) rather than globally when the client
+  opens
+- **Combine with Keyring unlock/lock:** Only start work when it is appropriate
+  for both UI open state and wallet state (e.g. client open **and** keyring
+  unlocked)
+- **Prefer pause/resume over stop/start for polling** so you can resume without
+  full re-initialization. Use the selector when subscribing (see example
+  below).
+
+```typescript
+import { clientControllerSelectors } from '@metamask/client-controller';
+
+class SomeDataController extends BaseController {
+  #uiOpen = false;
+  #keyringUnlocked = false;
+
+  constructor({ messenger }) {
+    super({ messenger, ... });
+
+    messenger.subscribe(
+      'ClientController:stateChange',
+      (isUiOpen) => {
+        this.#uiOpen = isUiOpen;
+        this.updateActive();
+      },
+      clientControllerSelectors.selectIsUiOpen,
+    );
+
+    messenger.subscribe('KeyringController:unlock', () => {
+      this.#keyringUnlocked = true;
+      this.updateActive();
+    });
+
+    messenger.subscribe('KeyringController:lock', () => {
+      this.#keyringUnlocked = false;
+      this.updateActive();
+    });
+  }
+
+  updateActive() {
+    const shouldRun = this.#uiOpen && this.#keyringUnlocked;
+    if (shouldRun) {
+      this.resume();
+    } else {
+      this.pause();
+    }
+  }
+}
+```
+
+Note: `stateChange` emits `[state, patches]`; the selector receives the full
+payload and returns the value passed to the handler (here, `isUiOpen`).
+
+## API Reference
+
+### State
+
+| Property   | Type      | Description                                |
+| ---------- | --------- | ------------------------------------------ |
+| `isUiOpen` | `boolean` | Whether the client (UI) is currently open. |
+
+State is not persisted. It always starts as `false`.
+
+### Actions
+
+| Action                       | Parameters      | Description                  |
+| ---------------------------- | --------------- | ---------------------------- |
+| `ClientController:getState`  | none            | Returns current state.       |
+| `ClientController:setUiOpen` | `open: boolean` | Sets whether the UI is open. |
+
+### Events
+
+| Event                          | Payload            | Description                  |
+| ------------------------------ | ------------------ | ---------------------------- |
+| `ClientController:stateChange` | `[state, patches]` | Standard state change event. |
+
+### Selectors
+
+```typescript
+import { clientControllerSelectors } from '@metamask/client-controller';
+
+const state = messenger.call('ClientController:getState');
+const isOpen = clientControllerSelectors.selectIsUiOpen(state);
+```
+
+## Contributing
+
+This package is part of a monorepo. Instructions for contributing can be found
+in the [monorepo README](https://github.com/MetaMask/core#readme).
diff --git a/packages/client-controller/jest.config.js b/packages/client-controller/jest.config.js
new file mode 100644
index 00000000000..9efbc1e7d1f
--- /dev/null
+++ b/packages/client-controller/jest.config.js
@@ -0,0 +1,24 @@
+/*
+ * For a detailed explanation regarding each configuration property and type check, visit:
+ * https://jestjs.io/docs/configuration
+ */
+
+const merge = require('deepmerge');
+const path = require('path');
+
+const baseConfig = require('../../jest.config.packages');
+
+const displayName = path.basename(__dirname);
+
+module.exports = merge(baseConfig, {
+  displayName,
+  coveragePathIgnorePatterns: [],
+  coverageThreshold: {
+    global: {
+      branches: 100,
+      functions: 100,
+      lines: 100,
+      statements: 100,
+    },
+  },
+});
diff --git a/packages/client-controller/package.json b/packages/client-controller/package.json
new file mode 100644
index 00000000000..44d62e64f2b
--- /dev/null
+++ b/packages/client-controller/package.json
@@ -0,0 +1,73 @@
+{
+  "name": "@metamask/client-controller",
+  "version": "0.0.0",
+  "description": "Client-level state for MetaMask (e.g. whether a UI window is open)",
+  "keywords": [
+    "MetaMask",
+    "Ethereum"
+  ],
+  "homepage": "https://github.com/MetaMask/core/tree/main/packages/client-controller#readme",
+  "bugs": {
+    "url": "https://github.com/MetaMask/core/issues"
+  },
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/MetaMask/core.git"
+  },
+  "license": "MIT",
+  "sideEffects": false,
+  "exports": {
+    ".": {
+      "import": {
+        "types": "./dist/index.d.mts",
+        "default": "./dist/index.mjs"
+      },
+      "require": {
+        "types": "./dist/index.d.cts",
+        "default": "./dist/index.cjs"
+      }
+    },
+    "./package.json": "./package.json"
+  },
+  "main": "./dist/index.cjs",
+  "types": "./dist/index.d.cts",
+  "files": [
+    "dist/"
+  ],
+  "scripts": {
+    "build": "ts-bridge --project tsconfig.build.json --verbose --clean --no-references",
+    "build:all": "ts-bridge --project tsconfig.build.json --verbose --clean",
+    "build:docs": "typedoc",
+    "changelog:update": "../../scripts/update-changelog.sh @metamask/client-controller",
+    "changelog:validate": "../../scripts/validate-changelog.sh @metamask/client-controller",
+    "publish:preview": "yarn npm publish --tag preview",
+    "since-latest-release": "../../scripts/since-latest-release.sh",
+    "test": "NODE_OPTIONS=--experimental-vm-modules jest --reporters=jest-silent-reporter",
+    "test:clean": "NODE_OPTIONS=--experimental-vm-modules jest --clearCache",
+    "test:verbose": "NODE_OPTIONS=--experimental-vm-modules jest --verbose",
+    "test:watch": "NODE_OPTIONS=--experimental-vm-modules jest --watch"
+  },
+  "dependencies": {
+    "@metamask/base-controller": "^9.0.0",
+    "@metamask/messenger": "^0.3.0"
+  },
+  "devDependencies": {
+    "@metamask/auto-changelog": "^3.4.4",
+    "@ts-bridge/cli": "^0.6.4",
+    "@types/jest": "^29.5.14",
+    "deepmerge": "^4.2.2",
+    "jest": "^29.7.0",
+    "jest-environment-jsdom": "^29.7.0",
+    "ts-jest": "^29.2.5",
+    "typedoc": "^0.25.13",
+    "typedoc-plugin-missing-exports": "^2.0.0",
+    "typescript": "~5.3.3"
+  },
+  "engines": {
+    "node": "^18.18 || >=20"
+  },
+  "publishConfig": {
+    "access": "public",
+    "registry": "https://registry.npmjs.org/"
+  }
+}
diff --git a/packages/client-controller/src/ClientController-method-action-types.ts b/packages/client-controller/src/ClientController-method-action-types.ts
new file mode 100644
index 00000000000..395b224ee67
--- /dev/null
+++ b/packages/client-controller/src/ClientController-method-action-types.ts
@@ -0,0 +1,25 @@
+/**
+ * This file is auto generated by `scripts/generate-method-action-types.ts`.
+ * Do not edit manually.
+ */
+
+import type { ClientController } from './ClientController';
+
+/**
+ * Updates state with whether the MetaMask UI is open.
+ *
+ * This method should be called when the user has opened the first window or
+ * screen containing the MetaMask UI, or closed the last window or screen
+ * containing the MetaMask UI.
+ *
+ * @param open - Whether the MetaMask UI is open.
+ */
+export type ClientControllerSetUiOpenAction = {
+  type: `ClientController:setUiOpen`;
+  handler: ClientController['setUiOpen'];
+};
+
+/**
+ * Union of all ClientController action types.
+ */
+export type ClientControllerMethodActions = ClientControllerSetUiOpenAction;
diff --git a/packages/client-controller/src/ClientController.test.ts b/packages/client-controller/src/ClientController.test.ts
new file mode 100644
index 00000000000..c7c7b7cf412
--- /dev/null
+++ b/packages/client-controller/src/ClientController.test.ts
@@ -0,0 +1,189 @@
+import { Messenger } from '@metamask/messenger';
+
+import type {
+  ClientControllerActions,
+  ClientControllerEvents,
+  ClientControllerMessenger,
+} from './ClientController';
+import {
+  ClientController,
+  controllerName,
+  getDefaultClientControllerState,
+} from './ClientController';
+import { clientControllerSelectors } from './selectors';
+
+describe('ClientController', () => {
+  type RootMessenger = Messenger<
+    'Root',
+    ClientControllerActions,
+    ClientControllerEvents
+  >;
+
+  /**
+   * Constructs the root messenger.
+   *
+   * @returns The root messenger.
+   */
+  function getRootMessenger(): RootMessenger {
+    return new Messenger<
+      'Root',
+      ClientControllerActions,
+      ClientControllerEvents
+    >({ namespace: 'Root' });
+  }
+
+  /**
+   * Constructs the messenger for the ClientController.
+   *
+   * @param rootMessenger - The root messenger.
+   * @returns The controller-specific messenger.
+   */
+  function getMessenger(
+    rootMessenger: RootMessenger,
+  ): ClientControllerMessenger {
+    return new Messenger<
+      typeof controllerName,
+      ClientControllerActions,
+      ClientControllerEvents,
+      RootMessenger
+    >({
+      namespace: controllerName,
+      parent: rootMessenger,
+    });
+  }
+
+  type WithControllerCallback<ReturnValue> = (payload: {
+    controller: ClientController;
+    rootMessenger: RootMessenger;
+    messenger: ClientControllerMessenger;
+  }) => Promise<ReturnValue> | ReturnValue;
+
+  type WithControllerOptions = {
+    options: Partial<ConstructorParameters<typeof ClientController>[0]>;
+  };
+
+  /**
+   * Wraps tests for the controller by creating the controller and messengers,
+   * then calling the test function with them.
+   *
+   * @param args - Either a callback, or an options bag + a callback. The
+   * options bag contains arguments for the controller constructor. The
+   * callback is called with the new controller, root messenger, and
+   * controller messenger.
+   * @returns The return value of the callback.
+   */
+  async function withController<ReturnValue>(
+    ...args:
+      | [WithControllerCallback<ReturnValue>]
+      | [WithControllerOptions, WithControllerCallback<ReturnValue>]
+  ): Promise<ReturnValue> {
+    const [{ options = {} }, testFunction] =
+      args.length === 2 ? args : [{}, args[0]];
+    const rootMessenger = getRootMessenger();
+    const messenger = getMessenger(rootMessenger);
+    const controller = new ClientController({
+      messenger,
+      ...options,
+    });
+    return await testFunction({ controller, rootMessenger, messenger });
+  }
+
+  describe('constructor', () => {
+    it('initializes with default state (client closed)', async () => {
+      await withController(({ controller }) => {
+        expect(controller.state).toMatchInlineSnapshot(`
+          {
+            "isUiOpen": false,
+          }
+        `);
+      });
+    });
+
+    it('allows initializing with partial state', async () => {
+      const givenState = { isUiOpen: true };
+      await withController(
+        { options: { state: givenState } },
+        ({ controller }) => {
+          expect(controller.state).toStrictEqual(givenState);
+        },
+      );
+    });
+
+    it('merges partial state with defaults', async () => {
+      await withController({ options: { state: {} } }, ({ controller }) => {
+        expect(controller.state).toMatchInlineSnapshot(`
+          {
+            "isUiOpen": false,
+          }
+        `);
+      });
+    });
+  });
+
+  describe('setUiOpen', () => {
+    it('updates isUiOpen in state to the given value', async () => {
+      await withController(({ controller }) => {
+        controller.setUiOpen(true);
+
+        expect(controller.state).toMatchInlineSnapshot(`
+          {
+            "isUiOpen": true,
+          }
+        `);
+
+        controller.setUiOpen(false);
+
+        expect(controller.state).toMatchInlineSnapshot(`
+          {
+            "isUiOpen": false,
+          }
+        `);
+      });
+    });
+  });
+
+  describe('messenger actions', () => {
+    it('allows setting client open via messenger action', async () => {
+      await withController(({ controller, messenger }) => {
+        messenger.call(`${controllerName}:setUiOpen`, true);
+        expect(controller.state).toStrictEqual({ isUiOpen: true });
+      });
+    });
+
+    it('allows setting client closed via messenger action', async () => {
+      await withController(({ controller, messenger }) => {
+        controller.setUiOpen(true);
+        messenger.call(`${controllerName}:setUiOpen`, false);
+        expect(controller.state).toStrictEqual({ isUiOpen: false });
+      });
+    });
+  });
+
+  describe('getDefaultClientControllerState', () => {
+    it('returns default state with client closed', () => {
+      const defaultState = getDefaultClientControllerState();
+
+      expect(defaultState.isUiOpen).toBe(false);
+    });
+  });
+
+  describe('selectors', () => {
+    describe('selectIsUiOpen', () => {
+      it('returns true when client is open', () => {
+        expect(
+          clientControllerSelectors.selectIsUiOpen({
+            isUiOpen: true,
+          }),
+        ).toBe(true);
+      });
+
+      it('returns false when client is closed', () => {
+        expect(
+          clientControllerSelectors.selectIsUiOpen({
+            isUiOpen: false,
+          }),
+        ).toBe(false);
+      });
+    });
+  });
+});
diff --git a/packages/client-controller/src/ClientController.ts b/packages/client-controller/src/ClientController.ts
new file mode 100644
index 00000000000..78553c1a990
--- /dev/null
+++ b/packages/client-controller/src/ClientController.ts
@@ -0,0 +1,212 @@
+import type {
+  StateMetadata,
+  ControllerGetStateAction,
+  ControllerStateChangeEvent,
+} from '@metamask/base-controller';
+import { BaseController } from '@metamask/base-controller';
+import type { Messenger } from '@metamask/messenger';
+
+import type { ClientControllerMethodActions } from './ClientController-method-action-types';
+
+// === GENERAL ===
+
+/**
+ * The name of the {@link ClientController}.
+ */
+export const controllerName = 'ClientController';
+
+// === STATE ===
+
+/**
+ * Describes the shape of the state object for {@link ClientController}.
+ */
+export type ClientControllerState = {
+  /**
+   * Whether the user has opened at least one window or screen
+   * containing the MetaMask UI. These windows or screens may or
+   * may not be in an inactive state.
+   */
+  isUiOpen: boolean;
+};
+
+/**
+ * Constructs the default {@link ClientController} state.
+ *
+ * @returns The default {@link ClientController} state.
+ */
+export function getDefaultClientControllerState(): ClientControllerState {
+  return {
+    isUiOpen: false,
+  };
+}
+
+/**
+ * The metadata for each property in {@link ClientControllerState}.
+ */
+const controllerMetadata = {
+  isUiOpen: {
+    includeInDebugSnapshot: true,
+    includeInStateLogs: true,
+    persist: false,
+    usedInUi: false,
+  },
+} satisfies StateMetadata<ClientControllerState>;
+
+// === MESSENGER ===
+
+const MESSENGER_EXPOSED_METHODS = ['setUiOpen'] as const;
+
+/**
+ * Retrieves the state of the {@link ClientController}.
+ */
+export type ClientControllerGetStateAction = ControllerGetStateAction<
+  typeof controllerName,
+  ClientControllerState
+>;
+
+/**
+ * Actions that {@link ClientController} exposes.
+ */
+export type ClientControllerActions =
+  | ClientControllerGetStateAction
+  | ClientControllerMethodActions;
+
+/**
+ * Actions from other messengers that {@link ClientController} calls.
+ */
+type AllowedActions = never;
+
+/**
+ * Published when the state of {@link ClientController} changes.
+ */
+export type ClientControllerStateChangeEvent = ControllerStateChangeEvent<
+  typeof controllerName,
+  ClientControllerState
+>;
+
+/**
+ * Events that {@link ClientController} exposes.
+ */
+export type ClientControllerEvents = ClientControllerStateChangeEvent;
+
+/**
+ * Events from other messengers that {@link ClientController} subscribes to.
+ */
+type AllowedEvents = never;
+
+/**
+ * The messenger for {@link ClientController}.
+ */
+export type ClientControllerMessenger = Messenger<
+  typeof controllerName,
+  ClientControllerActions | AllowedActions,
+  ClientControllerEvents | AllowedEvents
+>;
+
+// === CONTROLLER DEFINITION ===
+
+/**
+ * The options for constructing a {@link ClientController}.
+ */
+export type ClientControllerOptions = {
+  /**
+   * The messenger suited for this controller.
+   */
+  messenger: ClientControllerMessenger;
+  /**
+   * The initial state to set on this controller.
+   */
+  state?: Partial<ClientControllerState>;
+};
+
+/**
+ * `ClientController` manages the application lifecycle state.
+ *
+ * This controller tracks whether the MetaMask UI is open and publishes state
+ * change events that other controllers can subscribe to for adjusting their behavior.
+ *
+ * **Use cases:**
+ * - Polling controllers can pause when the UI closes, resume when it opens
+ * - WebSocket connections can disconnect when closed, reconnect when opened
+ * - Real-time subscriptions can pause when not visible
+ *
+ * **Platform Integration:**
+ * Platform code should call `ClientController:setUiOpen` via messenger.
+ *
+ * @example
+ * ```typescript
+ * // In MetamaskController or platform code
+ * onUiOpened() {
+ *   // ...
+ *   this.controllerMessenger.call('ClientController:setUiOpen', true);
+ * }
+ *
+ * onUiClosed() {
+ *   // ...
+ *   this.controllerMessenger.call('ClientController:setUiOpen', false);
+ * }
+ *
+ * // Consumer controller subscribing to state changes
+ * class MyController extends BaseController {
+ *   constructor({ messenger }) {
+ *     super({ messenger, ... });
+ *
+ *     messenger.subscribe(
+ *       'ClientController:stateChange',
+ *       (isClientOpen) => {
+ *         if (isClientOpen) {
+ *           this.resumePolling();
+ *         } else {
+ *           this.pausePolling();
+ *         }
+ *       },
+ *       clientControllerSelectors.selectIsUiOpen,
+ *     );
+ *   }
+ * }
+ * ```
+ */
+export class ClientController extends BaseController<
+  typeof controllerName,
+  ClientControllerState,
+  ClientControllerMessenger
+> {
+  /**
+   * Constructs a new {@link ClientController}.
+   *
+   * @param options - The constructor options.
+   * @param options.messenger - The messenger suited for this controller.
+   * @param options.state - The initial state to set on this controller.
+   */
+  constructor({ messenger, state = {} }: ClientControllerOptions) {
+    super({
+      messenger,
+      metadata: controllerMetadata,
+      name: controllerName,
+      state: {
+        ...getDefaultClientControllerState(),
+        ...state,
+      },
+    });
+
+    this.messenger.registerMethodActionHandlers(
+      this,
+      MESSENGER_EXPOSED_METHODS,
+    );
+  }
+
+  /**
+   * Updates state with whether the MetaMask UI is open.
+   *
+   * This method should be called when the user has opened the first window or
+   * screen containing the MetaMask UI, or closed the last window or screen
+   * containing the MetaMask UI.
+   *
+   * @param open - Whether the MetaMask UI is open.
+   */
+  setUiOpen(open: boolean): void {
+    this.update((state) => {
+      state.isUiOpen = open;
+    });
+  }
+}
diff --git a/packages/client-controller/src/index.ts b/packages/client-controller/src/index.ts
new file mode 100644
index 00000000000..8695331b716
--- /dev/null
+++ b/packages/client-controller/src/index.ts
@@ -0,0 +1,16 @@
+export {
+  ClientController,
+  getDefaultClientControllerState,
+} from './ClientController';
+export { clientControllerSelectors } from './selectors';
+
+export type {
+  ClientControllerState,
+  ClientControllerOptions,
+  ClientControllerGetStateAction,
+  ClientControllerActions,
+  ClientControllerStateChangeEvent,
+  ClientControllerEvents,
+  ClientControllerMessenger,
+} from './ClientController';
+export type { ClientControllerSetUiOpenAction } from './ClientController-method-action-types';
diff --git a/packages/client-controller/src/selectors.ts b/packages/client-controller/src/selectors.ts
new file mode 100644
index 00000000000..a8cefc6c5a3
--- /dev/null
+++ b/packages/client-controller/src/selectors.ts
@@ -0,0 +1,18 @@
+import type { ClientControllerState } from './ClientController';
+
+/**
+ * Selects whether the UI is currently open.
+ *
+ * @param state - The ClientController state.
+ * @returns True if the UI is open.
+ */
+const selectIsUiOpen = (state: ClientControllerState): boolean =>
+  state.isUiOpen;
+
+/**
+ * Selectors for the ClientController state.
+ * These can be used with Redux or directly with controller state.
+ */
+export const clientControllerSelectors = {
+  selectIsUiOpen,
+};
diff --git a/packages/client-controller/tsconfig.build.json b/packages/client-controller/tsconfig.build.json
new file mode 100644
index 00000000000..931c4d6594b
--- /dev/null
+++ b/packages/client-controller/tsconfig.build.json
@@ -0,0 +1,13 @@
+{
+  "extends": "../../tsconfig.packages.build.json",
+  "compilerOptions": {
+    "baseUrl": "./",
+    "outDir": "./dist",
+    "rootDir": "./src"
+  },
+  "references": [
+    { "path": "../base-controller/tsconfig.build.json" },
+    { "path": "../messenger/tsconfig.build.json" }
+  ],
+  "include": ["../../types", "./src"]
+}
diff --git a/packages/client-controller/tsconfig.json b/packages/client-controller/tsconfig.json
new file mode 100644
index 00000000000..3184a4bfde9
--- /dev/null
+++ b/packages/client-controller/tsconfig.json
@@ -0,0 +1,5 @@
+{
+  "extends": "../../tsconfig.packages.json",
+  "compilerOptions": { "baseUrl": "." },
+  "include": ["src"]
+}
diff --git a/packages/client-controller/typedoc.json b/packages/client-controller/typedoc.json
new file mode 100644
index 00000000000..d02905868c6
--- /dev/null
+++ b/packages/client-controller/typedoc.json
@@ -0,0 +1,8 @@
+{
+  "entryPoints": ["./src/index.ts"],
+  "excludePrivate": true,
+  "hideGenerator": true,
+  "out": "docs",
+  "plugin": ["typedoc-plugin-missing-exports"],
+  "tsconfig": "./tsconfig.build.json"
+}
diff --git a/teams.json b/teams.json
index 9e0811957fe..bb8112dce3b 100644
--- a/teams.json
+++ b/teams.json
@@ -35,6 +35,7 @@
   "metamask/multichain-api-middleware": "team-wallet-integrations",
   "metamask/selected-network-controller": "team-wallet-integrations",
   "metamask/eip-5792-middleware": "team-wallet-integrations",
+  "metamask/client-controller": "team-core-platform,team-extension-platform,team-mobile-platform",
   "metamask/base-controller": "team-core-platform",
   "metamask/build-utils": "team-core-platform",
   "metamask/composable-controller": "team-core-platform",
diff --git a/tsconfig.build.json b/tsconfig.build.json
index 8fa8236e1b1..ab53d9e4c52 100644
--- a/tsconfig.build.json
+++ b/tsconfig.build.json
@@ -58,6 +58,9 @@
     {
       "path": "./packages/claims-controller/tsconfig.build.json"
     },
+    {
+      "path": "./packages/client-controller/tsconfig.build.json"
+    },
     {
       "path": "./packages/composable-controller/tsconfig.build.json"
     },
diff --git a/tsconfig.json b/tsconfig.json
index 43acf48fbd2..509c82b30fc 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -59,6 +59,9 @@
     {
       "path": "./packages/claims-controller"
     },
+    {
+      "path": "./packages/client-controller"
+    },
     {
       "path": "./packages/composable-controller"
     },
diff --git a/yarn.lock b/yarn.lock
index a62b791a03f..e2ea8e7512b 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -2859,6 +2859,25 @@ __metadata:
   languageName: unknown
   linkType: soft
 
+"@metamask/client-controller@workspace:packages/client-controller":
+  version: 0.0.0-use.local
+  resolution: "@metamask/client-controller@workspace:packages/client-controller"
+  dependencies:
+    "@metamask/auto-changelog": "npm:^3.4.4"
+    "@metamask/base-controller": "npm:^9.0.0"
+    "@metamask/messenger": "npm:^0.3.0"
+    "@ts-bridge/cli": "npm:^0.6.4"
+    "@types/jest": "npm:^29.5.14"
+    deepmerge: "npm:^4.2.2"
+    jest: "npm:^29.7.0"
+    jest-environment-jsdom: "npm:^29.7.0"
+    ts-jest: "npm:^29.2.5"
+    typedoc: "npm:^0.25.13"
+    typedoc-plugin-missing-exports: "npm:^2.0.0"
+    typescript: "npm:~5.3.3"
+  languageName: unknown
+  linkType: soft
+
 "@metamask/composable-controller@workspace:packages/composable-controller":
   version: 0.0.0-use.local
   resolution: "@metamask/composable-controller@workspace:packages/composable-controller"