diff --git a/stdlib/builtins.pyi b/stdlib/builtins.pyi
index 87623557d..d78bbd453 100644
--- a/stdlib/builtins.pyi
+++ b/stdlib/builtins.pyi
@@ -1308,7 +1308,7 @@ def open(
 def open(
     file: _OpenFile,
     mode: OpenBinaryMode,
-    buffering: int,
+    buffering: int = ...,
     encoding: None = ...,
     errors: None = ...,
     newline: None = ...,
diff --git a/stdlib/importlib/abc.pyi b/stdlib/importlib/abc.pyi
index 1ff815858..0ac2a684d 100644
--- a/stdlib/importlib/abc.pyi
+++ b/stdlib/importlib/abc.pyi
@@ -158,7 +158,7 @@ if sys.version_info >= (3, 9):
         @overload
         @abstractmethod
         def open(
-            self, mode: OpenBinaryMode, buffering: int, encoding: None = ..., errors: None = ..., newline: None = ...
+            self, mode: OpenBinaryMode, buffering: int = ..., encoding: None = ..., errors: None = ..., newline: None = ...
         ) -> BinaryIO: ...
         # Fallback if mode is not specified
         @overload
diff --git a/stdlib/os/__init__.pyi b/stdlib/os/__init__.pyi
index 8f044c24e..8c284c8d1 100644
--- a/stdlib/os/__init__.pyi
+++ b/stdlib/os/__init__.pyi
@@ -570,7 +570,7 @@ def fdopen(
 def fdopen(
     fd: int,
     mode: OpenBinaryMode,
-    buffering: int,
+    buffering: int = ...,
     encoding: None = ...,
     errors: None = ...,
     newline: None = ...,
diff --git a/stdlib/pathlib.pyi b/stdlib/pathlib.pyi
index fc5d7d8fb..a0d43a468 100644
--- a/stdlib/pathlib.pyi
+++ b/stdlib/pathlib.pyi
@@ -147,7 +147,7 @@ class Path(PurePath):
     # Buffering cannot be determined: fall back to BinaryIO
     @overload
     def open(
-        self, mode: OpenBinaryMode, buffering: int, encoding: None = ..., errors: None = ..., newline: None = ...
+        self, mode: OpenBinaryMode, buffering: int = ..., encoding: None = ..., errors: None = ..., newline: None = ...
     ) -> BinaryIO: ...
     # Fallback if mode is not specified
     @overload
diff --git a/stubs/aiofiles/aiofiles/threadpool/__init__.pyi b/stubs/aiofiles/aiofiles/threadpool/__init__.pyi
index a43b5ab0d..1a5cb9bf8 100644
--- a/stubs/aiofiles/aiofiles/threadpool/__init__.pyi
+++ b/stubs/aiofiles/aiofiles/threadpool/__init__.pyi
@@ -86,7 +86,7 @@ def open(
 def open(
     file: _OpenFile,
     mode: OpenBinaryMode,
-    buffering: int,
+    buffering: int = ...,
     encoding: None = ...,
     errors: None = ...,
     newline: None = ...,