diff --git a/moto/iot/models.py b/moto/iot/models.py
index 1ebce5eaa..9ef417831 100644
--- a/moto/iot/models.py
+++ b/moto/iot/models.py
@@ -883,7 +883,8 @@ class IoTBackend(BaseBackend):
             if not do_merge:
                 thing.attributes = attributes
             else:
-                thing.attributes = {k: v for k, v in attributes.items() if v}
+                thing.attributes.update(attributes)
+            thing.attributes = {k: v for k, v in thing.attributes.items() if v}
 
     def create_keys_and_certificate(
         self, set_as_active: bool
diff --git a/tests/test_iot/test_iot_things.py b/tests/test_iot/test_iot_things.py
index 773bdab3e..ddaffdc66 100644
--- a/tests/test_iot/test_iot_things.py
+++ b/tests/test_iot/test_iot_things.py
@@ -52,7 +52,32 @@ def test_update_thing():
     assert len(res["things"]) == 1
     assert res["things"][0]["thingName"] is not None
     assert res["things"][0]["thingArn"] is not None
-    assert res["things"][0]["attributes"]["k1"] == "v1"
+    assert res["things"][0]["attributes"] == {"k1": "v1"}
+
+    client.update_thing(thingName=name, attributePayload={"attributes": {"k2": "v2"}})
+    res = client.list_things()
+    assert len(res["things"]) == 1
+    assert res["things"][0]["thingName"] is not None
+    assert res["things"][0]["thingArn"] is not None
+    assert res["things"][0]["attributes"] == {"k2": "v2"}
+
+    client.update_thing(
+        thingName=name, attributePayload={"attributes": {"k1": "v1"}, "merge": True}
+    )
+    res = client.list_things()
+    assert len(res["things"]) == 1
+    assert res["things"][0]["thingName"] is not None
+    assert res["things"][0]["thingArn"] is not None
+    assert res["things"][0]["attributes"] == {"k1": "v1", "k2": "v2"}
+
+    client.update_thing(
+        thingName=name, attributePayload={"attributes": {"k1": "v1.1"}, "merge": True}
+    )
+    res = client.list_things()
+    assert len(res["things"]) == 1
+    assert res["things"][0]["thingName"] is not None
+    assert res["things"][0]["thingArn"] is not None
+    assert res["things"][0]["attributes"] == {"k1": "v1.1", "k2": "v2"}
 
     client.update_thing(
         thingName=name, attributePayload={"attributes": {"k1": ""}, "merge": True}
@@ -61,6 +86,13 @@ def test_update_thing():
     assert len(res["things"]) == 1
     assert res["things"][0]["thingName"] is not None
     assert res["things"][0]["thingArn"] is not None
+    assert res["things"][0]["attributes"] == {"k2": "v2"}
+
+    client.update_thing(thingName=name, attributePayload={"attributes": {"k2": ""}})
+    res = client.list_things()
+    assert len(res["things"]) == 1
+    assert res["things"][0]["thingName"] is not None
+    assert res["things"][0]["thingArn"] is not None
     assert res["things"][0]["attributes"] == {}