What is MQTT? MQTT (Message Queuing Telemetry Transport) is a publish-subscribe based lightweight messaging protocol for use on top of the TCP/IP protocol. It was designed to be used in resource-constrained environments and for communication between devices with low-bandwidth or unreliable networks. MQTT is often used in Internet of Things (IoT) applications to communicate between devices and a central server, as well as in other types of messaging systems. In an MQTT system, there is a central broker that receives messages from publishers and routes them to subscribers that are subscribed to the relevant topic.