diff --git a/homeassistant/components/wirelesstag/sensor.py b/homeassistant/components/wirelesstag/sensor.py
index 09b70ded5ba0..061192248833 100644
--- a/homeassistant/components/wirelesstag/sensor.py
+++ b/homeassistant/components/wirelesstag/sensor.py
@@ -19,6 +19,7 @@ from homeassistant.helpers import config_validation as cv
 from homeassistant.helpers.dispatcher import async_dispatcher_connect
 from homeassistant.helpers.entity_platform import AddEntitiesCallback
 from homeassistant.helpers.typing import ConfigType, DiscoveryInfoType
+from homeassistant.util import slugify
 
 from . import WirelessTagPlatform
 from .const import DOMAIN, SIGNAL_TAG_UPDATE, WIRELESSTAG_DATA
@@ -115,8 +116,12 @@ class WirelessTagSensor(WirelessTagBaseSensor, SensorEntity):
         # I want to see entity_id as:
         # sensor.wirelesstag_bedroom_temperature
         # and not as sensor.bedroom for temperature and
-        # sensor.bedroom_2 for humidity
-        self.entity_id = f"sensor.{DOMAIN}_{self.underscored_name}_{self._sensor_type}"
+        # sensor.bedroom_2 for humidity.
+        # slugify ensures the entity_id stays valid for tag names containing
+        # accented/international or special characters.
+        self.entity_id = (
+            f"sensor.{slugify(f'{DOMAIN}_{self._tag.name}_{self._sensor_type}')}"
+        )
 
     @override
     async def async_added_to_hass(self) -> None:
@@ -129,11 +134,6 @@ class WirelessTagSensor(WirelessTagBaseSensor, SensorEntity):
             )
         )
 
-    @property
-    def underscored_name(self):
-        """Provide name savvy to be used in entity_id name of self."""
-        return self.name.lower().replace(" ", "_")
-
     @property
     @override
     def native_value(self):
diff --git a/tests/components/wirelesstag/test_sensor.py b/tests/components/wirelesstag/test_sensor.py
new file mode 100644
index 000000000000..0a301842038a
--- /dev/null
+++ b/tests/components/wirelesstag/test_sensor.py
@@ -0,0 +1,89 @@
+"""Tests for the Wireless Sensor Tags sensor platform."""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from homeassistant.const import PERCENTAGE, UnitOfTemperature
+from homeassistant.core import HomeAssistant, valid_entity_id
+from homeassistant.setup import async_setup_component
+
+CONFIG = {
+    "wirelesstag": {"username": "foo@bar.com", "password": "secret"},
+    "sensor": {
+        "platform": "wirelesstag",
+        "monitored_conditions": ["temperature", "humidity"],
+    },
+}
+
+
+def _mock_tag(name: str) -> MagicMock:
+    """Return a mocked wirelesstagpy SensorTag exposing temperature and humidity."""
+    tag = MagicMock()
+    tag.uuid = "00000000-0000-0000-0000-000000000001"
+    tag.tag_id = 1
+    tag.tag_manager_mac = "ABCDEF012345"
+    tag.name = name
+    tag.allowed_sensor_types = ["temperature", "humidity"]
+    tag.is_alive = True
+    tag.battery_remaining = 0.85
+    tag.battery_volts = 3.0
+    tag.signal_strength = -60
+    tag.is_in_range = True
+    tag.power_consumption = 1.5
+
+    def _sensor(sensor_type: str) -> MagicMock:
+        sensor = MagicMock()
+        if sensor_type == "temperature":
+            sensor.value = 21.5
+            sensor.unit = UnitOfTemperature.CELSIUS
+        else:
+            sensor.value = 45.0
+            sensor.unit = PERCENTAGE
+        return sensor
+
+    tag.sensor.__getitem__.side_effect = _sensor
+    return tag
+
+
+@pytest.mark.parametrize(
+    ("tag_name", "expected_entity_id"),
+    [
+        pytest.param(
+            "Bedroom",
+            "sensor.wirelesstag_bedroom_temperature",
+            id="ascii_name",
+        ),
+        pytest.param(
+            "Küche",
+            "sensor.wirelesstag_kuche_temperature",
+            id="non_ascii_name",
+        ),
+    ],
+)
+async def test_sensor_entity_id_is_valid(
+    hass: HomeAssistant,
+    caplog: pytest.LogCaptureFixture,
+    tag_name: str,
+    expected_entity_id: str,
+) -> None:
+    """Test sensors get a valid entity_id even for non-ASCII tag names.
+
+    A tag named e.g. "Küche" previously produced the invalid entity_id
+    "sensor.wirelesstag_küche_temperature", which is rejected by Home
+    Assistant's entity ID validation.
+    """
+    tag = _mock_tag(tag_name)
+    with patch("homeassistant.components.wirelesstag.WirelessTags") as mock_api_class:
+        mock_api = mock_api_class.return_value
+        mock_api.load_tags.return_value = {tag.uuid: tag}
+
+        assert await async_setup_component(hass, "wirelesstag", CONFIG)
+        await hass.async_block_till_done()
+        assert await async_setup_component(hass, "sensor", CONFIG)
+        await hass.async_block_till_done()
+
+    state = hass.states.get(expected_entity_id)
+    assert state is not None
+    assert valid_entity_id(state.entity_id)
+    assert "sets an invalid entity ID" not in caplog.text