diff --git a/moto/iot/models.py b/moto/iot/models.py
index 96078710125..a828a05f89e 100644
--- a/moto/iot/models.py
+++ b/moto/iot/models.py
@@ -331,6 +331,12 @@ def delete_thing(self, thing_name, expected_version):
 
         # can raise ResourceNotFoundError
         thing = self.describe_thing(thing_name)
+
+        # detach all principals
+        for k in list(self.principal_things.keys()):
+            if k[1] == thing_name:
+                del self.principal_things[k]
+
         del self.things[thing.arn]
 
     def delete_thing_type(self, thing_type_name):
diff --git a/tests/test_iot/test_iot.py b/tests/test_iot/test_iot.py
index a580f56d17a..23d4e78764f 100644
--- a/tests/test_iot/test_iot.py
+++ b/tests/test_iot/test_iot.py
@@ -519,6 +519,25 @@ def test_principal_thing():
     res.should.have.key('principals').which.should.have.length_of(0)
 
 
+@mock_iot
+def test_delete_principal_thing():
+    client = boto3.client('iot', region_name='ap-northeast-1')
+    thing_name = 'my-thing'
+    thing = client.create_thing(thingName=thing_name)
+    cert = client.create_keys_and_certificate(setAsActive=True)
+    cert_arn = cert['certificateArn']
+    cert_id = cert['certificateId']
+
+    client.attach_thing_principal(thingName=thing_name, principal=cert_arn)
+
+    client.delete_thing(thingName=thing_name)
+    res = client.list_principal_things(principal=cert_arn)
+    res.should.have.key('things').which.should.have.length_of(0)
+
+    client.update_certificate(certificateId=cert_id, newStatus="INACTIVE")
+    client.delete_certificate(certificateId=cert_id)
+
+
 @mock_iot
 def test_thing_groups():
     client = boto3.client('iot', region_name='ap-northeast-1')