diff --git a/src/sentry/integrations/github/integration.py b/src/sentry/integrations/github/integration.py
index 3a243bc05d4952..1bf90a7d286882 100644
--- a/src/sentry/integrations/github/integration.py
+++ b/src/sentry/integrations/github/integration.py
@@ -451,6 +451,8 @@ def sync_assignee_outbound(
 
             # Strip the @ from the username
             github_username = external_actor.external_name.lstrip("@")
+            # lowercase the username
+            github_username = github_username.lower()
 
         # Only update GitHub if we have a username to assign or if we're explicitly deassigning
         if github_username or not assign:
diff --git a/src/sentry/integrations/utils/sync.py b/src/sentry/integrations/utils/sync.py
index c4163386aa0505..4ff93355efff0e 100644
--- a/src/sentry/integrations/utils/sync.py
+++ b/src/sentry/integrations/utils/sync.py
@@ -139,7 +139,7 @@ def sync_group_assignee_inbound_by_external_actor(
 
         external_actors = ExternalActor.objects.filter(
             provider=EXTERNAL_PROVIDERS_REVERSE[ExternalProviderEnum(integration.provider)].value,
-            external_name=external_user_name,
+            external_name__iexact=external_user_name,
             integration_id=integration.id,
             user_id__isnull=False,
         ).values_list("user_id", flat=True)
diff --git a/tests/sentry/integrations/github/test_integration.py b/tests/sentry/integrations/github/test_integration.py
index fd98df4dd96789..dcab6a9889e792 100644
--- a/tests/sentry/integrations/github/test_integration.py
+++ b/tests/sentry/integrations/github/test_integration.py
@@ -1791,6 +1791,31 @@ def test_sync_assignee_outbound(self) -> None:
         assert request.url == "https://api.github.com/repos/Test-Organization/foo/issues/123"
         assert orjson.loads(request.body) == {"assignees": ["octocat"]}
 
+    @responses.activate
+    def test_sync_assignee_outbound_case_insensitive(self) -> None:
+        """Test assigning a GitHub issue to a user with linked GitHub account"""
+
+        user, installation, external_issue, _, _ = self._setup_assignee_sync_test(
+            external_name="@JohnDoe"
+        )
+
+        responses.add(
+            responses.PATCH,
+            "https://api.github.com/repos/Test-Organization/foo/issues/123",
+            json={"assignees": ["johndoe"]},
+            status=200,
+        )
+
+        responses.calls.reset()
+
+        with assume_test_silo_mode(SiloMode.REGION):
+            installation.sync_assignee_outbound(external_issue, user, assign=True)
+
+        assert len(responses.calls) == 1
+        request = responses.calls[0].request
+        assert request.url == "https://api.github.com/repos/Test-Organization/foo/issues/123"
+        assert orjson.loads(request.body) == {"assignees": ["johndoe"]}
+
     @responses.activate
     def test_sync_assignee_outbound_unassign(self) -> None:
         """Test unassigning a GitHub issue"""
diff --git a/tests/sentry/integrations/utils/test_sync.py b/tests/sentry/integrations/utils/test_sync.py
index 843329e1005cba..686993c8b2103b 100644
--- a/tests/sentry/integrations/utils/test_sync.py
+++ b/tests/sentry/integrations/utils/test_sync.py
@@ -345,6 +345,41 @@ def test_assignment_with_external_actor(
         assert updated_assignee.email == "test@example.com"
         mock_record_event.assert_called_with(EventLifecycleOutcome.SUCCESS, None, False, None)
 
+    @mock.patch("sentry.integrations.utils.metrics.EventLifecycle.record_event")
+    def test_assignment_with_external_actor_case_insensitive(
+        self,
+        mock_record_event: mock.MagicMock,
+    ) -> None:
+        """Test assigning a group to a user via external actor."""
+        assert self.group.get_assignee() is None
+
+        external_issue = self.create_integration_external_issue(
+            group=self.group,
+            key="JIRA-123",
+            integration=self.example_integration,
+        )
+
+        # Create external user mapping
+        self.create_external_user(
+            user=self.test_user,
+            external_name="@JohnDoe",
+            provider=ExternalProviders.GITHUB.value,
+            integration=self.example_integration,
+        )
+
+        sync_group_assignee_inbound_by_external_actor(
+            integration=self.example_integration,
+            external_user_name="@johndoe",
+            external_issue_key=external_issue.key,
+            assign=True,
+        )
+
+        updated_assignee = self.group.get_assignee()
+        assert updated_assignee is not None
+        assert updated_assignee.id == self.test_user.id
+        assert updated_assignee.email == "test@example.com"
+        mock_record_event.assert_called_with(EventLifecycleOutcome.SUCCESS, None, False, None)
+
     @mock.patch("sentry.integrations.utils.sync.where_should_sync")
     @mock.patch("sentry.integrations.utils.metrics.EventLifecycle.record_event")
     def test_assign_with_multiple_groups(