diff --git a/changelog.d/10175.bugfix b/changelog.d/10175.bugfix
new file mode 100644
index 000000000000..3679ada29689
--- /dev/null
+++ b/changelog.d/10175.bugfix
@@ -0,0 +1 @@
+Return an integer `expires_in` in requestOpenIdToken response.
diff --git a/synapse/rest/client/v2_alpha/openid.py b/synapse/rest/client/v2_alpha/openid.py
index d3322acc384b..e8d2673819cb 100644
--- a/synapse/rest/client/v2_alpha/openid.py
+++ b/synapse/rest/client/v2_alpha/openid.py
@@ -85,7 +85,7 @@ async def on_POST(self, request, user_id):
                 "access_token": token,
                 "token_type": "Bearer",
                 "matrix_server_name": self.server_name,
-                "expires_in": self.EXPIRES_MS / 1000,
+                "expires_in": self.EXPIRES_MS // 1000,
             },
         )