diff --git a/homeassistant/components/data_grand_lyon/config_flow.py b/homeassistant/components/data_grand_lyon/config_flow.py
index a5e0c50505d00..757e36cbe3f92 100644
--- a/homeassistant/components/data_grand_lyon/config_flow.py
+++ b/homeassistant/components/data_grand_lyon/config_flow.py
@@ -1,5 +1,6 @@
 """Config flow for the Data Grand Lyon integration."""
 
+from collections.abc import Mapping
 import logging
 from typing import Any
 
@@ -72,6 +73,36 @@ async def async_step_user(
             errors=errors,
         )
 
+    async def async_step_reauth(
+        self, entry_data: Mapping[str, Any]
+    ) -> ConfigFlowResult:
+        """Handle re-authentication."""
+        return await self.async_step_reauth_confirm()
+
+    async def async_step_reauth_confirm(
+        self, user_input: dict[str, Any] | None = None
+    ) -> ConfigFlowResult:
+        """Confirm re-authentication with new credentials."""
+        errors: dict[str, str] = {}
+        reauth_entry = self._get_reauth_entry()
+
+        if user_input is not None:
+            if error := await self._test_connection(user_input):
+                errors["base"] = error
+            else:
+                return self.async_update_reload_and_abort(
+                    reauth_entry, data_updates=user_input
+                )
+
+        return self.async_show_form(
+            step_id="reauth_confirm",
+            data_schema=self.add_suggested_values_to_schema(
+                STEP_USER_DATA_SCHEMA,
+                {CONF_USERNAME: reauth_entry.data[CONF_USERNAME]},
+            ),
+            errors=errors,
+        )
+
     async def _test_connection(self, user_input: dict[str, Any]) -> str | None:
         """Test connectivity by making a dummy API call.
 
diff --git a/homeassistant/components/data_grand_lyon/coordinator.py b/homeassistant/components/data_grand_lyon/coordinator.py
index 78e27258e33b9..e93b3f16f0f23 100644
--- a/homeassistant/components/data_grand_lyon/coordinator.py
+++ b/homeassistant/components/data_grand_lyon/coordinator.py
@@ -3,10 +3,12 @@
 import asyncio
 from datetime import timedelta
 
+from aiohttp import ClientResponseError
 from data_grand_lyon_ha import DataGrandLyonClient, TclPassage
 
 from homeassistant.config_entries import ConfigEntry
 from homeassistant.core import HomeAssistant
+from homeassistant.exceptions import ConfigEntryAuthFailed
 from homeassistant.helpers.update_coordinator import DataUpdateCoordinator, UpdateFailed
 
 from .const import CONF_LINE, CONF_STOP_ID, DOMAIN, LOGGER, SUBENTRY_TYPE_STOP
@@ -57,6 +59,13 @@ async def _async_update_data(self) -> dict[str, list[TclPassage]]:
         for i, subentry in enumerate(stop_subentries):
             result = stop_results[i]
             if isinstance(result, BaseException):
+                if isinstance(result, ClientResponseError) and result.status in (
+                    401,
+                    403,
+                ):
+                    raise ConfigEntryAuthFailed(
+                        "Authentication failed for Data Grand Lyon"
+                    ) from result
                 LOGGER.warning(
                     "Error fetching departures for stop %s: %s",
                     subentry.subentry_id,
diff --git a/homeassistant/components/data_grand_lyon/quality_scale.yaml b/homeassistant/components/data_grand_lyon/quality_scale.yaml
index 03e114feef493..6c739b099fb39 100644
--- a/homeassistant/components/data_grand_lyon/quality_scale.yaml
+++ b/homeassistant/components/data_grand_lyon/quality_scale.yaml
@@ -36,7 +36,7 @@ rules:
   integration-owner: done
   log-when-unavailable: done
   parallel-updates: done
-  reauthentication-flow: todo
+  reauthentication-flow: done
   test-coverage: done
 
   # Gold
diff --git a/homeassistant/components/data_grand_lyon/strings.json b/homeassistant/components/data_grand_lyon/strings.json
index 407108bf36caf..5b3a6b10f5c0b 100644
--- a/homeassistant/components/data_grand_lyon/strings.json
+++ b/homeassistant/components/data_grand_lyon/strings.json
@@ -1,7 +1,8 @@
 {
   "config": {
     "abort": {
-      "already_configured": "[%key:common::config_flow::abort::already_configured_service%]"
+      "already_configured": "[%key:common::config_flow::abort::already_configured_service%]",
+      "reauth_successful": "[%key:common::config_flow::abort::reauth_successful%]"
     },
     "error": {
       "cannot_connect": "[%key:common::config_flow::error::cannot_connect%]",
@@ -9,6 +10,16 @@
       "unknown": "[%key:common::config_flow::error::unknown%]"
     },
     "step": {
+      "reauth_confirm": {
+        "data": {
+          "password": "[%key:common::config_flow::data::password%]",
+          "username": "[%key:common::config_flow::data::username%]"
+        },
+        "data_description": {
+          "password": "[%key:component::data_grand_lyon::config::step::user::data_description::password%]",
+          "username": "[%key:component::data_grand_lyon::config::step::user::data_description::username%]"
+        }
+      },
       "user": {
         "data": {
           "password": "[%key:common::config_flow::data::password%]",
diff --git a/tests/components/data_grand_lyon/test_config_flow.py b/tests/components/data_grand_lyon/test_config_flow.py
index 784bde4e48d29..6c508f5fcfcb0 100644
--- a/tests/components/data_grand_lyon/test_config_flow.py
+++ b/tests/components/data_grand_lyon/test_config_flow.py
@@ -92,6 +92,85 @@ async def test_form_error_recovers(
     assert result["type"] is FlowResultType.CREATE_ENTRY
 
 
+async def test_reauth_flow(
+    hass: HomeAssistant,
+    mock_config_entry: MockConfigEntry,
+) -> None:
+    """Test the reauth flow updates credentials on success."""
+    mock_config_entry.add_to_hass(hass)
+
+    result = await mock_config_entry.start_reauth_flow(hass)
+    assert result["type"] is FlowResultType.FORM
+    assert result["step_id"] == "reauth_confirm"
+
+    with patch(
+        "homeassistant.components.data_grand_lyon.config_flow.DataGrandLyonClient.get_tcl_passages",
+        return_value=[],
+    ):
+        result = await hass.config_entries.flow.async_configure(
+            result["flow_id"],
+            {CONF_USERNAME: "new-user", CONF_PASSWORD: "new-pass"},
+        )
+
+    assert result["type"] is FlowResultType.ABORT
+    assert result["reason"] == "reauth_successful"
+    assert mock_config_entry.data == {
+        CONF_USERNAME: "new-user",
+        CONF_PASSWORD: "new-pass",
+    }
+
+
+@pytest.mark.parametrize(
+    ("side_effect", "error"),
+    [
+        (ClientConnectionError(), "cannot_connect"),
+        (ClientResponseError(None, None, status=401), "invalid_auth"),
+        (ClientResponseError(None, None, status=500), "cannot_connect"),
+        (RuntimeError("unexpected"), "unknown"),
+    ],
+    ids=["connection-error", "auth-401", "http-500", "unknown"],
+)
+async def test_reauth_flow_errors(
+    hass: HomeAssistant,
+    mock_config_entry: MockConfigEntry,
+    side_effect: Exception,
+    error: str,
+) -> None:
+    """Test the reauth flow shows errors and recovers."""
+    mock_config_entry.add_to_hass(hass)
+
+    result = await mock_config_entry.start_reauth_flow(hass)
+
+    with patch(
+        "homeassistant.components.data_grand_lyon.config_flow.DataGrandLyonClient.get_tcl_passages",
+        side_effect=side_effect,
+    ):
+        result = await hass.config_entries.flow.async_configure(
+            result["flow_id"],
+            {CONF_USERNAME: "new-user", CONF_PASSWORD: "new-pass"},
+        )
+
+    assert result["type"] is FlowResultType.FORM
+    assert result["step_id"] == "reauth_confirm"
+    assert result["errors"] == {"base": error}
+
+    with patch(
+        "homeassistant.components.data_grand_lyon.config_flow.DataGrandLyonClient.get_tcl_passages",
+        return_value=[],
+    ):
+        result = await hass.config_entries.flow.async_configure(
+            result["flow_id"],
+            {CONF_USERNAME: "new-user", CONF_PASSWORD: "new-pass"},
+        )
+
+    assert result["type"] is FlowResultType.ABORT
+    assert result["reason"] == "reauth_successful"
+    assert mock_config_entry.data == {
+        CONF_USERNAME: "new-user",
+        CONF_PASSWORD: "new-pass",
+    }
+
+
 async def test_form_already_configured(
     hass: HomeAssistant,
     mock_config_entry: MockConfigEntry,
diff --git a/tests/components/data_grand_lyon/test_init.py b/tests/components/data_grand_lyon/test_init.py
index 51138121de685..a73b4f2ccde9f 100644
--- a/tests/components/data_grand_lyon/test_init.py
+++ b/tests/components/data_grand_lyon/test_init.py
@@ -3,12 +3,15 @@
 from types import MappingProxyType
 from unittest.mock import AsyncMock
 
+from aiohttp import ClientResponseError
+
 from homeassistant.components.data_grand_lyon.const import (
     CONF_LINE,
     CONF_STOP_ID,
+    DOMAIN,
     SUBENTRY_TYPE_STOP,
 )
-from homeassistant.config_entries import ConfigSubentry
+from homeassistant.config_entries import SOURCE_REAUTH, ConfigEntryState, ConfigSubentry
 from homeassistant.core import HomeAssistant
 
 from tests.common import MockConfigEntry
@@ -42,3 +45,23 @@ async def test_subentry_added_reloads(
 
     assert mock_tcl_client.get_tcl_passages.call_count > initial_call_count
     assert hass.states.get("sensor.t1_stop_200_next_departure_1") is not None
+
+
+async def test_setup_triggers_reauth_on_auth_failure(
+    hass: HomeAssistant,
+    mock_config_entry: MockConfigEntry,
+    mock_tcl_client: AsyncMock,
+) -> None:
+    """Test that an auth failure during setup triggers a reauth flow."""
+    mock_tcl_client.get_tcl_passages.side_effect = ClientResponseError(
+        None, None, status=401
+    )
+
+    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 mock_config_entry.state is ConfigEntryState.SETUP_ERROR
+
+    flows = hass.config_entries.flow.async_progress_by_handler(DOMAIN)
+    assert any(flow["context"].get("source") == SOURCE_REAUTH for flow in flows)