diff --git a/mongoengine/queryset.py b/mongoengine/queryset.py
index c971d387..73a45299 100644
--- a/mongoengine/queryset.py
+++ b/mongoengine/queryset.py
@@ -721,7 +721,7 @@ class QuerySet(object):
         """Transform an update spec from Django-style format to Mongo format.
         """
         operators = ['set', 'unset', 'inc', 'dec', 'pop', 'push', 'push_all',
-                     'pull', 'pull_all']
+                     'pull', 'pull_all', 'add_to_set']
 
         mongo_update = {}
         for key, value in update.items():
@@ -739,7 +739,9 @@ class QuerySet(object):
                     op = 'inc'
                     if value > 0:
                         value = -value
-
+                elif op == 'add_to_set':
+                    op = op.replace('_to_set', 'ToSet')
+                    
             if _doc_cls:
                 # Switch field names to proper names [set in Field(name='foo')]
                 fields = QuerySet._lookup_field(_doc_cls, parts)
@@ -747,7 +749,8 @@ class QuerySet(object):
 
                 # Convert value to proper value
                 field = fields[-1]
-                if op in (None, 'set', 'unset', 'pop', 'push', 'pull'):
+                if op in (None, 'set', 'unset', 'pop', 'push', 'pull',
+                          'addToSet'):
                     value = field.prepare_query_value(op, value)
                 elif op in ('pushAll', 'pullAll'):
                     value = [field.prepare_query_value(op, v) for v in value]
diff --git a/tests/queryset.py b/tests/queryset.py
index 59a8216e..ab28ff3d 100644
--- a/tests/queryset.py
+++ b/tests/queryset.py
@@ -717,6 +717,11 @@ class QuerySetTest(unittest.TestCase):
         post.reload()
         self.assertEqual(post.tags, tags)
 
+        BlogPost.objects.update_one(add_to_set__tags='unique')
+        BlogPost.objects.update_one(add_to_set__tags='unique')
+        post.reload()
+        self.assertEqual(post.tags.count('unique'), 1)
+        
         BlogPost.drop_collection()
 
     def test_update_pull(self):