diff --git a/homeassistant/components/twentemilieu/diagnostics.py b/homeassistant/components/twentemilieu/diagnostics.py
new file mode 100644
index 00000000000..0d63fdbcf2c
--- /dev/null
+++ b/homeassistant/components/twentemilieu/diagnostics.py
@@ -0,0 +1,22 @@
+"""Diagnostics support for TwenteMilieu."""
+from __future__ import annotations
+
+from typing import Any
+
+from homeassistant.config_entries import ConfigEntry
+from homeassistant.const import CONF_ID
+from homeassistant.core import HomeAssistant
+from homeassistant.helpers.update_coordinator import DataUpdateCoordinator
+
+from .const import DOMAIN
+
+
+async def async_get_config_entry_diagnostics(
+    hass: HomeAssistant, entry: ConfigEntry
+) -> dict[str, Any]:
+    """Return diagnostics for a config entry."""
+    coordinator: DataUpdateCoordinator = hass.data[DOMAIN][entry.data[CONF_ID]]
+    return {
+        waste_type: waste_date.isoformat() if waste_date else None
+        for waste_type, waste_date in coordinator.data.items()
+    }
diff --git a/tests/components/twentemilieu/test_diagnostics.py b/tests/components/twentemilieu/test_diagnostics.py
new file mode 100644
index 00000000000..2f5ffcd5eb9
--- /dev/null
+++ b/tests/components/twentemilieu/test_diagnostics.py
@@ -0,0 +1,24 @@
+"""Tests for the diagnostics data provided by the TwenteMilieu integration."""
+from aiohttp import ClientSession
+
+from homeassistant.core import HomeAssistant
+
+from tests.common import MockConfigEntry
+from tests.components.diagnostics import get_diagnostics_for_config_entry
+
+
+async def test_diagnostics(
+    hass: HomeAssistant,
+    hass_client: ClientSession,
+    init_integration: MockConfigEntry,
+):
+    """Test diagnostics."""
+    assert await get_diagnostics_for_config_entry(
+        hass, hass_client, init_integration
+    ) == {
+        "0": "2021-11-01",
+        "1": "2021-11-02",
+        "2": None,
+        "6": "2022-01-06",
+        "10": "2021-11-03",
+    }