Can’t Connect to AWS IoT Core via MQTT in Postman (port 8883, TLS certs)

Question

Hi everyone, I’m currently working on a project using AWS IoT Core, and I want to test MQTT messages using Postman’s new MQTT client.
I’ve followed the AWS IoT setup:
Created a thing
Attached policy
Downloaded certificate, private key, and root CA
I’m using the following config in Postman:
Host: ........-ats.iot.ap-southeast-1.amazonaws.com
Port: 8883
Protocol: mqtts
Client cert and key: uploaded correctly
CA: Amazon Root CA 1
But I keep getting this error in Postman:
“Error: Connection refused”
I’ve confirmed it works in MQTT.fx with the same credentials, so I’m wondering:
Does Postman MQTT support TLS mutual authentication with AWS IoT Core? If so, what’s the correct way to set it up in Postman?

AWS-User-7851352 · Answer

If i understood your question correctly:

Postman’s new MQTT client does not currently support mutual TLS authentication (mTLS), which is a requirement for AWS IoT Core connections. Although you can upload certificates and keys in Postman, it seems that these are not correctly utilized for mTLS authentication like they are in MQTT.fx (which explicitly supports mTLS).

That’s why:

Your connection works in MQTT.fx (it fully supports mTLS).

Your connection **fails in Postman** (Postman’s MQTT implementation doesn’t support mTLS for client authentication yet).

What you can do instead:

Use MQTT.fx (or other mTLS-supported clients like `mosquitto_pub`/`mosquitto_sub`) for testing AWS IoT Core.
Use AWS IoT Core’s MQTT test client (via AWS Console) for simple testing.
Use the AWS CLI with `iot-data` commands for quick payload tests (though this bypasses the MQTT protocol).

Postman’s MQTT client is great for plain username/password or unsecured MQTT brokers, but not for AWS IoT Core’s strict mutual TLS setup.

Can’t Connect to AWS IoT Core via MQTT in Postman (port 8883, TLS certs)

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