diff --git a/homeassistant/components/stiebel_eltron/diagnostics.py b/homeassistant/components/stiebel_eltron/diagnostics.py
new file mode 100644
index 000000000000..483968dca5c6
--- /dev/null
+++ b/homeassistant/components/stiebel_eltron/diagnostics.py
@@ -0,0 +1,35 @@
+"""Diagnostics support for STIEBEL ELTRON."""
+
+from typing import Any
+
+from homeassistant.components.diagnostics import async_redact_data
+from homeassistant.const import CONF_HOST
+from homeassistant.core import HomeAssistant
+
+from . import StiebelEltronConfigEntry
+
+TO_REDACT = {CONF_HOST}
+
+
+async def async_get_config_entry_diagnostics(
+    hass: HomeAssistant, entry: StiebelEltronConfigEntry
+) -> dict[str, Any]:
+    """Return diagnostics for a config entry."""
+    coordinator = entry.runtime_data
+
+    return {
+        "entry_data": async_redact_data(entry.data, TO_REDACT),
+        "model": coordinator.device_info["model"],
+        "modbus": {
+            "is_connected": coordinator.api_client.is_connected,
+        },
+        "data": {
+            "current_temp": coordinator.api_client.get_current_temp(),
+            "target_temp": coordinator.api_client.get_target_temp(),
+            "current_humidity": coordinator.api_client.get_current_humidity(),
+            "operating_mode": coordinator.api_client.get_operation().name,
+            "heating_status": coordinator.api_client.get_heating_status(),
+            "cooling_status": coordinator.api_client.get_cooling_status(),
+            "filter_alarm": coordinator.api_client.get_filter_alarm_status(),
+        },
+    }
diff --git a/homeassistant/components/stiebel_eltron/quality_scale.yaml b/homeassistant/components/stiebel_eltron/quality_scale.yaml
index fb718177cb62..83242da57564 100644
--- a/homeassistant/components/stiebel_eltron/quality_scale.yaml
+++ b/homeassistant/components/stiebel_eltron/quality_scale.yaml
@@ -47,7 +47,7 @@ rules:
 
   # Gold
   devices: done
-  diagnostics: todo
+  diagnostics: done
   discovery-update-info: todo
   discovery: todo
   docs-data-update: todo
diff --git a/tests/components/stiebel_eltron/conftest.py b/tests/components/stiebel_eltron/conftest.py
index 5c43aab97196..489377acd783 100644
--- a/tests/components/stiebel_eltron/conftest.py
+++ b/tests/components/stiebel_eltron/conftest.py
@@ -44,9 +44,14 @@ def mock_lwz_api() -> Generator[MagicMock]:
         api_client.get_current_temp = MagicMock(return_value=21.0)
         api_client.get_current_humidity = MagicMock(return_value=45.0)
         api_client.get_operation = MagicMock(return_value=OperatingMode.AUTOMATIC)
+        api_client.get_heating_status = MagicMock(return_value=True)
+        api_client.get_cooling_status = MagicMock(return_value=False)
         api_client.get_filter_alarm_status = MagicMock(return_value=False)
 
-        api_client.connect = AsyncMock()
+        def _connect() -> None:
+            api_client.is_connected = True
+
+        api_client.connect = AsyncMock(side_effect=_connect)
         api_client.close = AsyncMock()
         api_client.async_update = AsyncMock()
         api_client.set_operation = AsyncMock()
diff --git a/tests/components/stiebel_eltron/snapshots/test_diagnostics.ambr b/tests/components/stiebel_eltron/snapshots/test_diagnostics.ambr
new file mode 100644
index 000000000000..004cc7fa8435
--- /dev/null
+++ b/tests/components/stiebel_eltron/snapshots/test_diagnostics.ambr
@@ -0,0 +1,22 @@
+# serializer version: 1
+# name: test_diagnostics
+  dict({
+    'data': dict({
+      'cooling_status': False,
+      'current_humidity': 45.0,
+      'current_temp': 21.0,
+      'filter_alarm': False,
+      'heating_status': True,
+      'operating_mode': 'AUTOMATIC',
+      'target_temp': 22.5,
+    }),
+    'entry_data': dict({
+      'host': '**REDACTED**',
+      'port': 502,
+    }),
+    'modbus': dict({
+      'is_connected': True,
+    }),
+    'model': 'LWZ',
+  })
+# ---
diff --git a/tests/components/stiebel_eltron/test_diagnostics.py b/tests/components/stiebel_eltron/test_diagnostics.py
new file mode 100644
index 000000000000..bc1c820fb1d0
--- /dev/null
+++ b/tests/components/stiebel_eltron/test_diagnostics.py
@@ -0,0 +1,26 @@
+"""Tests for STIEBEL ELTRON diagnostics."""
+
+from syrupy.assertion import SnapshotAssertion
+
+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_diagnostics(
+    hass: HomeAssistant,
+    hass_client: ClientSessionGenerator,
+    mock_config_entry: MockConfigEntry,
+    snapshot: SnapshotAssertion,
+) -> None:
+    """Test config entry diagnostics."""
+    mock_config_entry.add_to_hass(hass)
+    assert 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
+    )