diff --git a/models.py b/models.py
index dd10ea9..60aa444 100644
--- a/models.py
+++ b/models.py
@@ -1,6 +1,6 @@
 """Datastore model classes."""
 import base64
-from datetime import timezone
+from datetime import timedelta, timezone
 import difflib
 import itertools
 import logging
@@ -36,6 +36,19 @@ PROTOCOLS = ('activitypub', 'bluesky', 'ostatus', 'webmention', 'ui')
 KEY_BITS = 1024 if DEBUG else 2048
 PAGE_SIZE = 20
 
+# auto delete old objects of these types via the Object.expire property
+# https://cloud.google.com/datastore/docs/ttl
+OBJECT_EXPIRE_TYPES = (
+    'post',
+    'update',
+    'delete',
+    'accept',
+    'reject',
+    'undo',
+    None
+)
+OBJECT_EXPIRE_AGE = timedelta(days=90)
+
 logger = logging.getLogger(__name__)
 
 
@@ -331,7 +344,6 @@ class Object(StringIdModel):
     def _object_ids(self):  # id(s) of inner objects
         if self.as1:
             return common.redirect_unwrap(as1.get_ids(self.as1, 'object'))
-
     object_ids = ndb.ComputedProperty(_object_ids, repeated=True)
 
     deleted = ndb.BooleanProperty()
@@ -343,6 +355,16 @@ class Object(StringIdModel):
     created = ndb.DateTimeProperty(auto_now_add=True)
     updated = ndb.DateTimeProperty(auto_now=True)
 
+    # For certain types, automatically delete this Object after 90d using a
+    # TTL policy:
+    # https://cloud.google.com/datastore/docs/ttl#ttl_properties_and_indexes
+    # They recommend not indexing TTL properties:
+    # https://cloud.google.com/datastore/docs/ttl#ttl_properties_and_indexes
+    def _expire(self):
+        if self.type in OBJECT_EXPIRE_TYPES:
+            return (self.updated or util.now()) + OBJECT_EXPIRE_AGE
+    expire = ndb.ComputedProperty(_expire, indexed=False)
+
     def _pre_put_hook(self):
         assert '^^' not in self.key.id()
 
diff --git a/tests/test_models.py b/tests/test_models.py
index 40afa37..7b3caa2 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -4,11 +4,11 @@ from unittest import mock
 
 from flask import g, get_flashed_messages
 from granary import as2
-from oauth_dropins.webutil.testutil import requests_response
+from oauth_dropins.webutil.testutil import NOW, requests_response
 
 from app import app
 import common
-from models import Follower, Object, User
+from models import Follower, Object, OBJECT_EXPIRE_AGE, User
 import protocol
 from protocol import Protocol
 from . import testutil
@@ -329,6 +329,10 @@ class ObjectTest(testutil.TestCase):
     def test_computed_properties_without_as1(self):
         Object(id='a').put()
 
+    def test_expire(self):
+        obj = Object(id='a', our_as1={'objectType': 'activity', 'verb': 'update'})
+        self.assertEqual(NOW + OBJECT_EXPIRE_AGE, obj.expire)
+
     def test_put_adds_removes_activity_label(self):
         obj = Object(id='x#y', our_as1={})
         obj.put()
diff --git a/tests/testutil.py b/tests/testutil.py
index 704672c..ac35a87 100644
--- a/tests/testutil.py
+++ b/tests/testutil.py
@@ -182,8 +182,8 @@ class TestCase(unittest.TestCase, testutil.Asserts):
             got.mf2.pop('url', None)
 
         self.assert_entities_equal(Object(id=id, **props), got,
-                                   ignore=['as1', 'created', 'object_ids',
-                                           'type', 'updated'])
+                                   ignore=['as1', 'created', 'expire',
+                                           'object_ids', 'type', 'updated'])
 
     def assert_equals(self, expected, actual, msg=None, ignore=(), **kwargs):
         return super().assert_equals(