diff --git a/stdlib/io.pyi b/stdlib/io.pyi
index 3e9a6cd6861d..2d3aadc483f0 100644
--- a/stdlib/io.pyi
+++ b/stdlib/io.pyi
@@ -103,7 +103,7 @@ class FileIO(RawIOBase, BinaryIO):
     def __enter__(self: Self) -> Self: ...
 
 class BytesIO(BufferedIOBase, BinaryIO):
-    def __init__(self, initial_bytes: bytes = ...) -> None: ...
+    def __init__(self, initial_bytes: ReadableBuffer = ...) -> None: ...
     # BytesIO does not contain a "name" field. This workaround is necessary
     # to allow BytesIO sub-classes to add this field, as it is defined
     # as a read-only property on IO[].