diff --git a/Lib/base64.py b/Lib/base64.py
index e8bc252ecfa638..6636e06382f4de 100755
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -607,7 +607,14 @@ def main():
         with open(args[0], 'rb') as f:
             func(f, sys.stdout.buffer)
     else:
-        func(sys.stdin.buffer, sys.stdout.buffer)
+        if sys.stdin.isatty():
+            # gh-138775: read terminal input data all at once to detect EOF
+            import io
+            data = sys.stdin.buffer.read()
+            buffer = io.BytesIO(data)
+        else:
+            buffer = sys.stdin.buffer
+        func(buffer, sys.stdout.buffer)
 
 
 if __name__ == '__main__':
diff --git a/Misc/NEWS.d/next/Library/2025-09-11-15-03-37.gh-issue-138775.w7rnSx.rst b/Misc/NEWS.d/next/Library/2025-09-11-15-03-37.gh-issue-138775.w7rnSx.rst
new file mode 100644
index 00000000000000..455c1a9925a5e1
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-09-11-15-03-37.gh-issue-138775.w7rnSx.rst
@@ -0,0 +1,2 @@
+Use of ``python -m`` with :mod:`base64` has been fixed to detect input from a
+terminal so that it properly notices EOF.