diff --git a/homeassistant/components/data_grand_lyon/diagnostics.py b/homeassistant/components/data_grand_lyon/diagnostics.py
new file mode 100644
index 0000000000000..87379d38b6e45
--- /dev/null
+++ b/homeassistant/components/data_grand_lyon/diagnostics.py
@@ -0,0 +1,27 @@
+"""Diagnostics support for the Data Grand Lyon integration."""
+
+from dataclasses import asdict
+from typing import Any
+
+from homeassistant.components.diagnostics import async_redact_data
+from homeassistant.const import CONF_PASSWORD, CONF_USERNAME
+from homeassistant.core import HomeAssistant
+
+from .coordinator import DataGrandLyonConfigEntry
+
+TO_REDACT = {CONF_USERNAME, CONF_PASSWORD}
+
+
+async def async_get_config_entry_diagnostics(
+    hass: HomeAssistant, entry: DataGrandLyonConfigEntry
+) -> dict[str, Any]:
+    """Return diagnostics for a config entry."""
+    coordinator = entry.runtime_data
+
+    return {
+        "config_entry": async_redact_data(entry.as_dict(), TO_REDACT),
+        "coordinator_data": {
+            subentry_id: [asdict(passage) for passage in passages]
+            for subentry_id, passages in coordinator.data.items()
+        },
+    }
diff --git a/homeassistant/components/data_grand_lyon/quality_scale.yaml b/homeassistant/components/data_grand_lyon/quality_scale.yaml
index 2e2522901f0e3..fa9357574073b 100644
--- a/homeassistant/components/data_grand_lyon/quality_scale.yaml
+++ b/homeassistant/components/data_grand_lyon/quality_scale.yaml
@@ -41,7 +41,7 @@ rules:
 
   # Gold
   devices: done
-  diagnostics: todo
+  diagnostics: done
   discovery-update-info:
     status: exempt
     comment: This is a service integration; there are no discoverable devices.
diff --git a/tests/components/data_grand_lyon/snapshots/test_diagnostics.ambr b/tests/components/data_grand_lyon/snapshots/test_diagnostics.ambr
new file mode 100644
index 0000000000000..8d1acdc6e5a60
--- /dev/null
+++ b/tests/components/data_grand_lyon/snapshots/test_diagnostics.ambr
@@ -0,0 +1,59 @@
+# serializer version: 1
+# name: test_config_entry_diagnostics
+  dict({
+    'config_entry': dict({
+      'data': dict({
+        'password': '**REDACTED**',
+        'username': '**REDACTED**',
+      }),
+      'disabled_by': None,
+      'discovery_keys': dict({
+      }),
+      'domain': 'data_grand_lyon',
+      'minor_version': 1,
+      'options': dict({
+      }),
+      'pref_disable_new_entities': False,
+      'pref_disable_polling': False,
+      'source': 'user',
+      'subentries': list([
+        dict({
+          'data': dict({
+            'line': 'C3',
+            'stop_id': 100,
+          }),
+          'subentry_type': 'stop',
+          'title': 'C3 - Stop 100',
+          'unique_id': 'C3_100',
+        }),
+      ]),
+      'title': 'Data Grand Lyon',
+      'unique_id': None,
+      'version': 1,
+    }),
+    'coordinator_data': dict({
+      'stop_1': list([
+        dict({
+          'course_theorique': 'A',
+          'delai_passage': '3 min',
+          'direction': 'Gare Part-Dieu',
+          'heure_passage': '2026-04-10T14:03:00',
+          'id': 100,
+          'id_tarret_destination': 0,
+          'ligne': 'C3',
+          'type': 'E',
+        }),
+        dict({
+          'course_theorique': 'B',
+          'delai_passage': '8 min',
+          'direction': 'Gare St-Paul',
+          'heure_passage': '2026-04-10T14:08:00',
+          'id': 100,
+          'id_tarret_destination': 0,
+          'ligne': 'C3',
+          'type': 'T',
+        }),
+      ]),
+    }),
+  })
+# ---
diff --git a/tests/components/data_grand_lyon/test_diagnostics.py b/tests/components/data_grand_lyon/test_diagnostics.py
new file mode 100644
index 0000000000000..c5c412d739133
--- /dev/null
+++ b/tests/components/data_grand_lyon/test_diagnostics.py
@@ -0,0 +1,29 @@
+"""Tests for the Data Grand Lyon diagnostics."""
+
+from unittest.mock import AsyncMock
+
+from syrupy.assertion import SnapshotAssertion
+from syrupy.filters import props
+
+from homeassistant.core import HomeAssistant
+
+from tests.common import MockConfigEntry
+from tests.components.diagnostics import get_diagnostics_for_config_entry
+from tests.typing import ClientSessionGenerator
+
+
+async def test_config_entry_diagnostics(
+    hass: HomeAssistant,
+    hass_client: ClientSessionGenerator,
+    mock_config_entry: MockConfigEntry,
+    mock_tcl_client: AsyncMock,
+    snapshot: SnapshotAssertion,
+) -> None:
+    """Test config entry diagnostics."""
+    mock_config_entry.add_to_hass(hass)
+    await hass.config_entries.async_setup(mock_config_entry.entry_id)
+    await hass.async_block_till_done()
+
+    assert await get_diagnostics_for_config_entry(
+        hass, hass_client, mock_config_entry
+    ) == snapshot(exclude=props("created_at", "modified_at", "entry_id", "subentry_id"))