diff --git a/stdlib/3/io.pyi b/stdlib/3/io.pyi
index 167385318..a22955bb1 100644
--- a/stdlib/3/io.pyi
+++ b/stdlib/3/io.pyi
@@ -255,5 +255,6 @@ class StringIO(TextIOWrapper):
                  newline: Optional[str] = ...) -> None: ...
     name = ...  # type: str
     def getvalue(self) -> str: ...
+    def __enter__(self) -> 'StringIO': ...
 
 class IncrementalNewlineDecoder(codecs.IncrementalDecoder): ...