diff --git a/moto/kms/responses.py b/moto/kms/responses.py
index 8cd6e766..53012b7f 100644
--- a/moto/kms/responses.py
+++ b/moto/kms/responses.py
@@ -238,7 +238,7 @@ class KmsResponse(BaseResponse):
 
         value = self.parameters.get("CiphertextBlob")
         try:
-            return json.dumps({"Plaintext": base64.b64decode(value).decode("utf-8")})
+            return json.dumps({"Plaintext": base64.b64decode(value).decode("utf-8"), 'KeyId': 'key_id'})
         except UnicodeDecodeError:
             # Generate data key will produce random bytes which when decrypted is still returned as base64
             return json.dumps({"Plaintext": value})
diff --git a/tests/test_kms/test_kms.py b/tests/test_kms/test_kms.py
index 8fe0620f..f189fbe4 100644
--- a/tests/test_kms/test_kms.py
+++ b/tests/test_kms/test_kms.py
@@ -191,6 +191,7 @@ def test_decrypt():
     conn = boto.kms.connect_to_region('us-west-2')
     response = conn.decrypt('ZW5jcnlwdG1l'.encode('utf-8'))
     response['Plaintext'].should.equal(b'encryptme')
+    response['KeyId'].should.equal('key_id')
 
 
 @mock_kms_deprecated