Caution

This library is not maintained. It was my first 'popular' Open Source project, and brought MQTT to the Arduino platform just as both were gaining popularity.

But times have changed, and I haven't needed to use it for a long time now. This means I simply don't have the setup locally to properly develop, test and maintain the library. Alternative libraries exist and I urge you to pick one that is actively maintained.

So Long and Thanks For All The PUBACKs

Nick 2026

Arduino Client for MQTT

This library provides a client for doing simple publish/subscribe messaging with a server that supports MQTT.

Examples

The library comes with a number of example sketches. See File > Examples > PubSubClient within the Arduino application.

Full API documentation is available here: https://pubsubclient.knolleary.net

Limitations

• It can only publish QoS 0 messages. It can subscribe at QoS 0 or QoS 1.
• The maximum message size, including header, is 256 bytes by default. This is configurable via MQTT_MAX_PACKET_SIZE in PubSubClient.h or can be changed by calling PubSubClient::setBufferSize(size).
• The keepalive interval is set to 15 seconds by default. This is configurable via MQTT_KEEPALIVE in PubSubClient.h or can be changed by calling PubSubClient::setKeepAlive(keepAlive).
• The client uses MQTT 3.1.1 by default. It can be changed to use MQTT 3.1 by changing value of MQTT_VERSION in PubSubClient.h.

Compatible Hardware

The library uses the Arduino Ethernet Client api for interacting with the underlying network hardware. This means it Just Works with a growing number of boards and shields, including:

• Arduino Ethernet
• Arduino Ethernet Shield
• Arduino YUN – use the included YunClient in place of EthernetClient, and be sure to do a Bridge.begin() first
• Arduino WiFi Shield - if you want to send packets > 90 bytes with this shield, enable the MQTT_MAX_TRANSFER_SIZE define in PubSubClient.h.
• Sparkfun WiFly Shield – library
• TI CC3000 WiFi - library
• Intel Galileo/Edison
• ESP8266
• ESP32

The library cannot currently be used with hardware based on the ENC28J60 chip – such as the Nanode or the Nuelectronics Ethernet Shield. For those, there is an alternative library available.

License

This code is released under the MIT License.