diff --git a/Misc/NEWS.d/next/Library/2021-11-16-18-13-49.bpo-41735.D72UY1.rst b/Misc/NEWS.d/next/Library/2021-11-16-18-13-49.bpo-41735.D72UY1.rst
new file mode 100644
index 00000000000000..101da0e9ce6487
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-11-16-18-13-49.bpo-41735.D72UY1.rst
@@ -0,0 +1 @@
+Fix thread lock in ``zlib.Decompress.flush()`` method before ``PyObject_GetBuffer``.
diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
index 68de701efa45f1..a6940f2fd43a3f 100644
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -1266,12 +1266,13 @@ zlib_Decompress_flush_impl(compobject *self, PyTypeObject *cls,
         return NULL;
     }
 
+    ENTER_ZLIB(self);
+
     if (PyObject_GetBuffer(self->unconsumed_tail, &data, PyBUF_SIMPLE) == -1) {
+        LEAVE_ZLIB(self);
         return NULL;
     }
 
-    ENTER_ZLIB(self);
-
     self->zst.next_in = data.buf;
     ibuflen = data.len;