diff --git a/homeassistant/core.py b/homeassistant/core.py
index e7f4f8758f8..31bb281aeaa 100644
--- a/homeassistant/core.py
+++ b/homeassistant/core.py
@@ -32,7 +32,7 @@ from homeassistant.const import (
     EVENT_SERVICE_REMOVED, __version__)
 from homeassistant import loader
 from homeassistant.exceptions import (
-    HomeAssistantError, InvalidEntityFormatError)
+    HomeAssistantError, InvalidEntityFormatError, InvalidStateError)
 from homeassistant.util.async import (
     run_coroutine_threadsafe, run_callback_threadsafe,
     fire_coroutine_threadsafe)
@@ -65,6 +65,11 @@ def valid_entity_id(entity_id: str) -> bool:
     return ENTITY_ID_PATTERN.match(entity_id) is not None
 
 
+def valid_state(state: str) -> bool:
+    """Test if an state is valid."""
+    return len(state) < 256
+
+
 def callback(func: Callable[..., None]) -> Callable[..., None]:
     """Annotation to mark method as safe to call from within the event loop."""
     # pylint: disable=protected-access
@@ -520,13 +525,20 @@ class State(object):
     def __init__(self, entity_id, state, attributes=None, last_changed=None,
                  last_updated=None):
         """Initialize a new state."""
+        state = str(state)
+
         if not valid_entity_id(entity_id):
             raise InvalidEntityFormatError((
                 "Invalid entity id encountered: {}. "
                 "Format should be <domain>.<object_id>").format(entity_id))
 
+        if not valid_state(state):
+            raise InvalidStateError((
+                "Invalid state encountered for entity id: {}. "
+                "State max length is 255 characters.").format(entity_id))
+
         self.entity_id = entity_id.lower()
-        self.state = str(state)
+        self.state = state
         self.attributes = MappingProxyType(attributes or {})
         self.last_updated = last_updated or dt_util.utcnow()
         self.last_changed = last_changed or self.last_updated
diff --git a/homeassistant/exceptions.py b/homeassistant/exceptions.py
index 2889d83af5c..cb8a3c87820 100644
--- a/homeassistant/exceptions.py
+++ b/homeassistant/exceptions.py
@@ -32,3 +32,9 @@ class PlatformNotReady(HomeAssistantError):
     """Error to indicate that platform is not ready."""
 
     pass
+
+
+class InvalidStateError(HomeAssistantError):
+    """When an invalid state is encountered."""
+
+    pass
diff --git a/tests/test_core.py b/tests/test_core.py
index b7d049cb747..c3fea749f5d 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -12,7 +12,8 @@ import pytz
 import pytest
 
 import homeassistant.core as ha
-from homeassistant.exceptions import InvalidEntityFormatError
+from homeassistant.exceptions import (InvalidEntityFormatError,
+                                      InvalidStateError)
 from homeassistant.util.async import run_coroutine_threadsafe
 import homeassistant.util.dt as dt_util
 from homeassistant.util.unit_system import (METRIC_SYSTEM)
@@ -421,6 +422,10 @@ class TestState(unittest.TestCase):
             InvalidEntityFormatError, ha.State,
             'invalid_entity_format', 'test_state')
 
+        self.assertRaises(
+            InvalidStateError, ha.State,
+            'domain.long_state', 't' * 256)
+
     def test_domain(self):
         """Test domain."""
         state = ha.State('some_domain.hello', 'world')