diff --git a/Lib/gzip.py b/Lib/gzip.py
index 8d21fe4bc24..f934d4f1c2b 100644
--- a/Lib/gzip.py
+++ b/Lib/gzip.py
@@ -96,7 +96,7 @@ class _PaddedFile:
             self._read -= len(prepend)
             return
         else:
-            self._buffer = self._buffer[read:] + prepend
+            self._buffer = self._buffer[self._read:] + prepend
         self._length = len(self._buffer)
         self._read = 0
 
diff --git a/Lib/test/test_gzip.py b/Lib/test/test_gzip.py
index 52894077c9c..b7a7e03c96a 100644
--- a/Lib/test/test_gzip.py
+++ b/Lib/test/test_gzip.py
@@ -421,6 +421,13 @@ class TestGzip(BaseTest):
         with gzip.GzipFile(fileobj=io.BytesIO(gzdata)) as f:
             self.assertEqual(f.read(), b'Test')
 
+    def test_prepend_error(self):
+        # See issue #20875
+        with gzip.open(self.filename, "wb") as f:
+            f.write(data1)
+        with gzip.open(self.filename, "rb") as f:
+            f.fileobj.prepend()
+
 class TestOpen(BaseTest):
     def test_binary_modes(self):
         uncompressed = data1 * 50
diff --git a/Misc/NEWS b/Misc/NEWS
index 4f7328cc22f..68cb3790abb 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -20,6 +20,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #20875: Prevent possible gzip "'read' is not defined" NameError.
+  Patch by Claudiu Popa.
+
 - Issue #11558: ``email.message.Message.attach`` now returns a more
   useful error message if ``attach`` is called on a message for which
   ``is_multipart`` is False.