diff --git a/Lib/base64.py b/Lib/base64.py
index 5d78cc09f40cd3..4107290aead1a5 100644
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -601,7 +601,14 @@ 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 input data at once when reading from stdin.
+            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..a01ab6c2619dea
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-09-11-15-03-37.gh-issue-138775.w7rnSx.rst
@@ -0,0 +1 @@
+:mod:`base64`: fix the EOF being ignored in command-line interface.