diff --git a/moto/emr/models.py b/moto/emr/models.py
index 79b1080c2..93651db90 100644
--- a/moto/emr/models.py
+++ b/moto/emr/models.py
@@ -332,6 +332,15 @@ class FakeCluster(BaseModel):
             if self.master_instance_group_id:
                 raise Exception("Cannot add another master instance group")
             self.master_instance_group_id = instance_group.id
+            num_master_nodes = instance_group.num_instances
+            if num_master_nodes > 1:
+                # Cluster is HA
+                if num_master_nodes != 3:
+                    raise ValidationException(
+                        "Master instance group must have exactly 3 instances for HA clusters."
+                    )
+                self.keep_job_flow_alive_when_no_steps = True
+                self.termination_protected = True
         if instance_group.role == "CORE":
             if self.core_instance_group_id:
                 raise Exception("Cannot add another core instance group")
diff --git a/tests/test_emr/test_emr_boto3.py b/tests/test_emr/test_emr_boto3.py
index 61ad89756..2c8a44b47 100644
--- a/tests/test_emr/test_emr_boto3.py
+++ b/tests/test_emr/test_emr_boto3.py
@@ -1126,3 +1126,60 @@ def test_security_configurations():
     ex.value.response["Error"]["Message"].should.match(
         r"Security configuration with name .* does not exist."
     )
+
+
+@mock_emr
+def test_run_job_flow_with_invalid_number_of_master_nodes_raises_error():
+    client = boto3.client("emr", region_name="us-east-1")
+    params = dict(
+        Name="test-cluster",
+        Instances={
+            "InstanceGroups": [
+                {
+                    "InstanceCount": 2,
+                    "InstanceRole": "MASTER",
+                    "InstanceType": "c1.medium",
+                    "Market": "ON_DEMAND",
+                    "Name": "master",
+                }
+            ]
+        },
+    )
+    with pytest.raises(ClientError) as ex:
+        client.run_job_flow(**params)
+    error = ex.value.response["Error"]
+    error["Code"].should.equal("ValidationException")
+    error["Message"].should.equal(
+        "Master instance group must have exactly 3 instances for HA clusters."
+    )
+
+
+@mock_emr
+def test_run_job_flow_with_multiple_master_nodes():
+    client = boto3.client("emr", region_name="us-east-1")
+    params = dict(
+        Name="test-cluster",
+        Instances={
+            "InstanceGroups": [
+                {
+                    "InstanceCount": 3,
+                    "InstanceRole": "MASTER",
+                    "InstanceType": "c1.medium",
+                    "Market": "ON_DEMAND",
+                    "Name": "master",
+                }
+            ],
+            "KeepJobFlowAliveWhenNoSteps": False,
+            "TerminationProtected": False,
+        },
+    )
+    cluster_id = client.run_job_flow(**params)["JobFlowId"]
+    cluster = client.describe_cluster(ClusterId=cluster_id)["Cluster"]
+    cluster["AutoTerminate"].should.equal(False)
+    cluster["TerminationProtected"].should.equal(True)
+    groups = client.list_instance_groups(ClusterId=cluster_id)["InstanceGroups"]
+    master_instance_group = next(
+        group for group in groups if group["InstanceGroupType"] == "MASTER"
+    )
+    master_instance_group["RequestedInstanceCount"].should.equal(3)
+    master_instance_group["RunningInstanceCount"].should.equal(3)