diff --git a/moto/ec2/exceptions.py b/moto/ec2/exceptions.py
index 8db419354..0aebec08a 100644
--- a/moto/ec2/exceptions.py
+++ b/moto/ec2/exceptions.py
@@ -233,10 +233,13 @@ class InvalidRouteError(EC2ClientError):
 
 class InvalidInstanceIdError(EC2ClientError):
     def __init__(self, instance_id):
-        super().__init__(
-            "InvalidInstanceID.NotFound",
-            "The instance ID '{0}' does not exist".format(instance_id),
-        )
+        if isinstance(instance_id, str):
+            instance_id = [instance_id]
+        if len(instance_id) > 1:
+            msg = f"The instance IDs '{', '.join(instance_id)}' do not exist"
+        else:
+            msg = f"The instance ID '{instance_id[0]}' does not exist"
+        super().__init__("InvalidInstanceID.NotFound", msg)
 
 
 class InvalidInstanceTypeError(EC2ClientError):
diff --git a/moto/ec2/models.py b/moto/ec2/models.py
index e8d5b2b36..7681b380a 100644
--- a/moto/ec2/models.py
+++ b/moto/ec2/models.py
@@ -1284,9 +1284,10 @@ class InstanceBackend(object):
                     if instance.applies(filters):
                         result.append(instance)
 
-        # TODO: Trim error message down to specific invalid id.
         if instance_ids and len(instance_ids) > len(result):
-            raise InvalidInstanceIdError(instance_ids)
+            result_ids = [i.id for i in result]
+            missing_instance_ids = [i for i in instance_ids if i not in result_ids]
+            raise InvalidInstanceIdError(missing_instance_ids)
 
         return result
 
diff --git a/tests/test_ec2/test_instances.py b/tests/test_ec2/test_instances.py
index b05078e2d..2813f8932 100644
--- a/tests/test_ec2/test_instances.py
+++ b/tests/test_ec2/test_instances.py
@@ -2049,6 +2049,43 @@ def test_instance_termination_protection():
     instance["State"]["Name"].should.equal("terminated")
 
 
+@mock_ec2
+def test_terminate_unknown_instances():
+    client = boto3.client("ec2", region_name="us-west-1")
+
+    # Correct error message for single unknown instance
+    with pytest.raises(ClientError) as ex:
+        client.terminate_instances(InstanceIds=["i-12345678"])
+    error = ex.value.response["Error"]
+    error["Code"].should.equal("InvalidInstanceID.NotFound")
+    error["Message"].should.equal("The instance ID 'i-12345678' does not exist")
+
+    # Correct error message for multiple unknown instances
+    with pytest.raises(ClientError) as ex:
+        client.terminate_instances(InstanceIds=["i-12345678", "i-12345668"])
+    error = ex.value.response["Error"]
+    error["Code"].should.equal("InvalidInstanceID.NotFound")
+    error["Message"].should.equal(
+        "The instance IDs 'i-12345678, i-12345668' do not exist"
+    )
+
+    # Create an instance
+    resp = client.run_instances(ImageId=EXAMPLE_AMI_ID, MinCount=1, MaxCount=1)
+    instance_id = resp["Instances"][0]["InstanceId"]
+
+    # Correct error message if one instance is known
+    with pytest.raises(ClientError) as ex:
+        client.terminate_instances(InstanceIds=["i-12345678", instance_id])
+    error = ex.value.response["Error"]
+    error["Code"].should.equal("InvalidInstanceID.NotFound")
+    error["Message"].should.equal("The instance ID 'i-12345678' does not exist")
+
+    # status = still running
+    resp = client.describe_instances(InstanceIds=[instance_id])
+    instance = resp["Reservations"][0]["Instances"][0]
+    instance["State"]["Name"].should.equal("running")
+
+
 @mock_ec2
 def test_instance_lifecycle():
     ec2_resource = boto3.resource("ec2", "us-west-1")