diff --git a/stix2/core.py b/stix2/core.py
index 8ee11f5..20bd187 100644
--- a/stix2/core.py
+++ b/stix2/core.py
@@ -7,9 +7,9 @@ from .base import _STIXBase
 from .common import MarkingDefinition
 from .properties import IDProperty, ListProperty, Property, TypeProperty
 from .sdo import (AttackPattern, Campaign, CourseOfAction, Identity, Indicator,
-                  IntrusionSet, Malware, ObservedData, Report, ThreatActor,
-                  Tool, Vulnerability)
-from .sro import Relationship, Sighting
+                  IntrusionSet, Malware, ObservedData, Report,
+                  STIXDomainObject, ThreatActor, Tool, Vulnerability)
+from .sro import Relationship, Sighting, STIXRelationshipObject
 from .utils import get_dict
 
 
@@ -20,6 +20,11 @@ class STIXObjectProperty(Property):
         super(STIXObjectProperty, self).__init__()
 
     def clean(self, value):
+        # Any STIX Object (SDO, SRO, or Marking Definition) can be added to
+        # a bundle with no further checks.
+        if isinstance(value, (STIXDomainObject, STIXRelationshipObject,
+                              MarkingDefinition)):
+            return value
         try:
             dictified = get_dict(value)
         except ValueError:
diff --git a/stix2/test/test_bundle.py b/stix2/test/test_bundle.py
index c7c95a8..12b2149 100644
--- a/stix2/test/test_bundle.py
+++ b/stix2/test/test_bundle.py
@@ -158,3 +158,11 @@ def test_parse_unknown_type():
     with pytest.raises(stix2.exceptions.ParseError) as excinfo:
         stix2.parse(unknown)
     assert str(excinfo.value) == "Can't parse unknown object type 'other'! For custom types, use the CustomObject decorator."
+
+
+def test_stix_object_property():
+    prop = stix2.core.STIXObjectProperty()
+
+    identity = stix2.Identity(name="test", identity_class="individual")
+    assert prop.clean(identity) == identity
+    assert prop.clean(identity) is identity