diff --git a/homeassistant/helpers/entity.py b/homeassistant/helpers/entity.py
index 187d53ea00bcb7..b5cc9f973f6044 100644
--- a/homeassistant/helpers/entity.py
+++ b/homeassistant/helpers/entity.py
@@ -429,6 +429,8 @@ def _stringify_state(self) -> str:
             # If the entity's state is a float, limit precision according to machine
             # epsilon to make the string representation readable
             return f"{state:.{FLOAT_PRECISION}}"
+        if isinstance(state, datetime):
+            return state.isoformat()
         return str(state)
 
     @callback
diff --git a/homeassistant/helpers/typing.py b/homeassistant/helpers/typing.py
index 7d01b0b6a77fbf..99f25d1c83adb6 100644
--- a/homeassistant/helpers/typing.py
+++ b/homeassistant/helpers/typing.py
@@ -1,4 +1,5 @@
 """Typing Helpers for Home Assistant."""
+from datetime import datetime
 from enum import Enum
 from typing import Any, Dict, Mapping, Optional, Tuple, Union
 
@@ -10,7 +11,7 @@
 DiscoveryInfoType = Dict[str, Any]
 EventType = homeassistant.core.Event
 ServiceDataType = Dict[str, Any]
-StateType = Union[None, str, int, float]
+StateType = Union[None, str, int, float, datetime]
 TemplateVarsType = Optional[Mapping[str, Any]]
 
 # Custom type for recorder Queries
diff --git a/tests/helpers/test_entity.py b/tests/helpers/test_entity.py
index 8142f563f0172f..aad80dfa1be47e 100644
--- a/tests/helpers/test_entity.py
+++ b/tests/helpers/test_entity.py
@@ -1,7 +1,7 @@
 """Test the entity helper."""
 # pylint: disable=protected-access
 import asyncio
-from datetime import timedelta
+from datetime import datetime, timedelta, timezone
 import threading
 from unittest.mock import MagicMock, PropertyMock, patch
 
@@ -788,3 +788,17 @@ async def test_float_conversion(hass):
     state = hass.states.get("hello.world")
     assert state is not None
     assert state.state == "3.6"
+
+
+async def test_datetime_conversion(hass):
+    """Test conversion of datetime state to ISO 8601 datetime strings."""
+    object_state = datetime(2017, 12, 19, 18, 29, 42, tzinfo=timezone.utc)
+    with patch.object(entity.Entity, "state", PropertyMock(return_value=object_state)):
+        ent = entity.Entity()
+        ent.hass = hass
+        ent.entity_id = "hello.world"
+        ent.async_write_ha_state()
+
+    state = hass.states.get("hello.world")
+    assert state is not None
+    assert state.state == "2017-12-19T18:29:42+00:00"