Publish an MQTT message

Use this action to publish a message to an MQTT topic. A common use is to send a command to a device that listens on MQTT, for example switching a light or relay, or to publish a value other systems can subscribe to.

Using this action from the user interface

If you prefer building automations and scripts visually, Home Assistant walks you through this action step by step. You pick what to target, tweak a few options, and save. No YAML knowledge required.

To publish a message from an automation or a script:

1. Go to Settings > Automations & scenes.
2. Open an existing automation or script, or select Create automation > Create new automation.
3. If you’re setting up a new automation, add a trigger in the When section. Scripts don’t need a trigger. They run when something else calls them.
4. In the Then do section, select Add action.
5. Search for and select Publish.
6. Enter the Topic to publish to.
7. Optionally, enter the Payload to send. Under Publish options, you can also set the quality of service, the retain flag, and other options.
8. Select Save.

Options in the UI

Using this action in YAML

If you work directly in YAML, or you want to know exactly what Home Assistant does under the hood, this section has the technical reference. It lists the field names you use in YAML, their types, and which ones are required.

In YAML, refer to this action as mqtt.publish. A basic example looks like this:

action: mqtt.publish
data:
  topic: "homeassistant/light/1/command"
  payload: "ON"

Options in YAML

Good to know

• The payload must be a string. The MQTT integration supports templates, so you can build the payload from the state of your entities.

• To publish JSON, format and escape it as a string. A folded block keeps it readable:

  topic: "homeassistant/light/1/state"
  payload: >-
    {"Status": "off", "Data": "something"}
  

• When the payload is a Python bytes literal and you want to publish raw data instead of text, turn on Evaluate payload.

• Publishing an empty message with Retain turned on clears a previously retained message on that topic.

More examples

Real scenarios where this action shows up in automations and scripts. Copy any example and adapt it to your setup.

Automation: turn on a device when you get home

Send a command to a device over MQTT as soon as you arrive home.

• Trigger: You arrive home
• Action: Publish an MQTT message

alias: "Turn on the porch light when I get home"
triggers:
  - trigger: state
    entity_id: person.me
    to: "home"
actions:
  - action: mqtt.publish
    data:
      topic: "homeassistant/light/porch/command"
      payload: "ON"
      retain: true

Publish an MQTT discovery configuration

You can use this action to set up a device through MQTT discovery by publishing a configuration message. The example below sets up a temperature sensor.

action: mqtt.publish
data:
  topic: "homeassistant/sensor/bathroom_temperature/config"
  payload: >-
    {"device_class": "temperature",
    "unit_of_measurement": "\u00b0C",
    "value_template": "{{ value | float }}",
    "state_topic": "sensors/bathroom/temperature",
    "unique_id": "bathroom_temperature",
    "device": {
    "identifiers": "bathroom_sensor",
    "name": "Bathroom",
    "manufacturer": "rtl_433" }
    }

Still stuck?

The Home Assistant community is quick to help: join Discord for real-time chat, post on the community forum with the action you’re calling and what you expected to happen, or share on our subreddit /r/homeassistant.

Related actions

These actions work well alongside this one:

• Export MQTT messages: Writes all messages on a topic to a file for debugging.

• Reload MQTT entities: Reloads MQTT entities from your YAML configuration.