diff --git a/moto/ssm/models.py b/moto/ssm/models.py
index bb25baa5f..fc74e1524 100644
--- a/moto/ssm/models.py
+++ b/moto/ssm/models.py
@@ -144,7 +144,7 @@ class SimpleSystemManagerBackend(BaseBackend):
     def send_command(self, **kwargs):
         instances = kwargs.get('InstanceIds', [])
         now = datetime.datetime.now()
-        expires_after = now + datetime.timedelta(0, int(kwargs['TimeoutSeconds']))
+        expires_after = now + datetime.timedelta(0, int(kwargs.get('TimeoutSeconds', 3600)))
         return {
             'Command': {
                 'CommandId': str(uuid.uuid4()),
diff --git a/tests/test_ssm/test_ssm_boto3.py b/tests/test_ssm/test_ssm_boto3.py
index 97801e0b9..0531d1780 100644
--- a/tests/test_ssm/test_ssm_boto3.py
+++ b/tests/test_ssm/test_ssm_boto3.py
@@ -481,7 +481,6 @@ def test_send_command():
     response = client.send_command(
         InstanceIds=['i-123456'],
         DocumentName=ssm_document,
-        TimeoutSeconds=60,
         Parameters=params,
         OutputS3Region='us-east-2',
         OutputS3BucketName='the-bucket',