diff --git a/NEWS b/NEWS
index 3144cdf88c59..83cdbf662489 100644
--- a/NEWS
+++ b/NEWS
@@ -195,6 +195,8 @@ PHP                                                                        NEWS
   . Allowed filtered streams to be casted as fd for select. (Jakub Zelenka)
   . Fixed bug GH-21221 (Prevent closing of innerstream of php://temp stream).
     (ilutov)
+  . Fixed bug GH-21700 (Assertion failure in _php_stream_seek when a user
+    stream returns a negative position). (lacatoire)
   . Improved stream_socket_server() bind failure error reporting. (ilutov)
   . Fixed bug #49874 (ftell() and fseek() inconsistency when using stream
     filters). (Jakub Zelenka)
diff --git a/ext/standard/tests/streams/gh21700_user_stream_append_negative_position.phpt b/ext/standard/tests/streams/gh21700_user_stream_append_negative_position.phpt
new file mode 100644
index 000000000000..f3037e3bd389
--- /dev/null
+++ b/ext/standard/tests/streams/gh21700_user_stream_append_negative_position.phpt
@@ -0,0 +1,27 @@
+--TEST--
+GH-21700: Append-mode open ignores a negative position returned by a user stream
+--FILE--
+<?php
+class lyingstream {
+    public function stream_open($path, $mode, $options, &$opened_path) { return true; }
+    public function stream_seek($offset, $whence) { return true; }
+    public function stream_tell(): int { return -42; }
+    public function stream_eof() { return false; }
+    public function stream_read($count) { return ''; }
+}
+
+stream_wrapper_register("test", "lyingstream");
+
+// Opening in append mode triggers an internal SEEK_CUR to revise the initial
+// position; if the user stream lies, ftell() must still report a non-negative
+// value and a subsequent SEEK_CUR must not trip the position invariant.
+$fp = fopen("test://foo", "ab");
+var_dump(ftell($fp));
+var_dump(fseek($fp, 0, SEEK_CUR));
+fclose($fp);
+
+stream_wrapper_unregister("test");
+?>
+--EXPECT--
+int(0)
+int(-1)
diff --git a/ext/standard/tests/streams/gh21700_user_stream_negative_position.phpt b/ext/standard/tests/streams/gh21700_user_stream_negative_position.phpt
new file mode 100644
index 000000000000..fe9ea64a86a2
--- /dev/null
+++ b/ext/standard/tests/streams/gh21700_user_stream_negative_position.phpt
@@ -0,0 +1,48 @@
+--TEST--
+GH-21700: User stream returning negative position must not trigger an assertion
+--FILE--
+<?php
+class mystream {
+    public int $pos = 0;
+    public function stream_open($path, $mode, $options, &$opened_path) { return true; }
+    public function stream_seek($offset, $whence) {
+        // Blindly store the offset, mimicking a buggy user implementation.
+        $this->pos = $offset;
+        return true;
+    }
+    public function stream_tell() { return $this->pos; }
+    public function stream_eof() { return false; }
+    public function stream_read($count) { return ''; }
+}
+
+class lyingstream extends mystream {
+    public function stream_tell(): int { return -42; }
+}
+
+// 1. SEEK_CUR with a negative resulting absolute position is rejected
+//    before invoking the user seek hook, so stream->position stays valid.
+stream_wrapper_register("test", "mystream");
+$fp = fopen("test://foo", "rb");
+var_dump(fseek($fp, -1, SEEK_CUR));
+var_dump(ftell($fp));
+var_dump(fseek($fp, 0, SEEK_CUR));
+fclose($fp);
+stream_wrapper_unregister("test");
+
+// 2. A user stream_tell() returning a negative position is rejected,
+//    keeping stream->position non-negative.
+stream_wrapper_register("test", "lyingstream");
+$fp = fopen("test://foo", "rb");
+var_dump(fseek($fp, 100, SEEK_SET));
+var_dump(ftell($fp));
+var_dump(fseek($fp, 0, SEEK_CUR));
+fclose($fp);
+stream_wrapper_unregister("test");
+?>
+--EXPECT--
+int(-1)
+int(0)
+int(0)
+int(-1)
+int(0)
+int(-1)
diff --git a/main/streams/streams.c b/main/streams/streams.c
index 31d1eda16790..e097064ecb75 100644
--- a/main/streams/streams.c
+++ b/main/streams/streams.c
@@ -1466,19 +1466,31 @@ PHPAPI int _php_stream_seek(php_stream *stream, zend_off_t offset, int whence)
 				} else {
 					offset = stream->position + offset;
 				}
- 				whence = SEEK_SET;
-				break;
+				whence = SEEK_SET;
+				ZEND_FALLTHROUGH;
 			case SEEK_SET:
 				if (offset < 0) {
 					return -1;
 				}
+				break;
 		}
+		zend_off_t saved_position = stream->position;
 		ret = stream->ops->seek(stream, offset, whence, &stream->position);
 
 		if (((stream->flags & PHP_STREAM_FLAG_NO_SEEK) == 0) || ret == 0) {
 			if (ret == 0) {
-				stream->eof = 0;
-				stream->fatal_error = 0;
+				if (UNEXPECTED(stream->position < 0)) {
+					/* The seek operation reported success but updated
+					 * stream->position to a negative value (e.g. a user
+					 * stream's stream_tell() returned a negative offset).
+					 * Treat as failure and restore the original position
+					 * to keep the invariant stream->position >= 0. */
+					stream->position = saved_position;
+					ret = -1;
+				} else {
+					stream->eof = 0;
+					stream->fatal_error = 0;
+				}
 			}
 
 			/* invalidate the buffer contents */
@@ -2392,7 +2404,7 @@ PHPAPI php_stream *_php_stream_open_wrapper_ex(const char *path, const char *mod
 		zend_off_t newpos = 0;
 
 		/* if opened for append, we need to revise our idea of the initial file position */
-		if (0 == stream->ops->seek(stream, 0, SEEK_CUR, &newpos)) {
+		if (0 == stream->ops->seek(stream, 0, SEEK_CUR, &newpos) && newpos >= 0) {
 			stream->position = newpos;
 		}
 	}