diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 9739b6d37fb21b..342a50724d49fb 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -1928,7 +1928,7 @@ def truncate(self, pos=None):
         """Truncate size to pos, where pos is an int."""
         self._unsupported("truncate")
 
-    def readline(self):
+    def readline(self, size=-1, /):
         """Read until newline or EOF.
 
         Returns an empty string if EOF is hit immediately.
diff --git a/Misc/NEWS.d/next/Library/2026-06-18-23-59-46.gh-issue-151596.5ma144.rst b/Misc/NEWS.d/next/Library/2026-06-18-23-59-46.gh-issue-151596.5ma144.rst
new file mode 100644
index 00000000000000..17ec1341142e93
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-18-23-59-46.gh-issue-151596.5ma144.rst
@@ -0,0 +1 @@
+Add missing ``size`` positional argument to the pure-Python implementation of :meth:`io.TextIOBase.readline`.