diff --git a/homeassistant/components/light/mqtt.py b/homeassistant/components/light/mqtt.py
index 1c01278197d..92030c8617a 100644
--- a/homeassistant/components/light/mqtt.py
+++ b/homeassistant/components/light/mqtt.py
@@ -211,7 +211,11 @@ class MqttLight(MqttAvailability, MqttDiscoveryUpdate, Light):
         self._optimistic_rgb = \
             optimistic or topic[CONF_RGB_STATE_TOPIC] is None
         self._optimistic_brightness = (
-            optimistic or topic[CONF_BRIGHTNESS_STATE_TOPIC] is None)
+            optimistic or
+            (topic[CONF_BRIGHTNESS_COMMAND_TOPIC] is not None and
+             topic[CONF_BRIGHTNESS_STATE_TOPIC] is None) or
+            (topic[CONF_BRIGHTNESS_COMMAND_TOPIC] is None and
+             topic[CONF_RGB_STATE_TOPIC] is None))
         self._optimistic_color_temp = (
             optimistic or topic[CONF_COLOR_TEMP_STATE_TOPIC] is None)
         self._optimistic_effect = (
@@ -233,7 +237,8 @@ class MqttLight(MqttAvailability, MqttDiscoveryUpdate, Light):
         self._white_value = None
         self._supported_features = 0
         self._supported_features |= (
-            topic[CONF_RGB_COMMAND_TOPIC] is not None and SUPPORT_COLOR)
+            topic[CONF_RGB_COMMAND_TOPIC] is not None and
+            (SUPPORT_COLOR | SUPPORT_BRIGHTNESS))
         self._supported_features |= (
             topic[CONF_BRIGHTNESS_COMMAND_TOPIC] is not None and
             SUPPORT_BRIGHTNESS)
@@ -325,6 +330,10 @@ class MqttLight(MqttAvailability, MqttDiscoveryUpdate, Light):
 
             rgb = [int(val) for val in payload.split(',')]
             self._hs = color_util.color_RGB_to_hs(*rgb)
+            if self._topic[CONF_BRIGHTNESS_STATE_TOPIC] is None:
+                percent_bright = \
+                    float(color_util.color_RGB_to_hsv(*rgb)[2]) / 100.0
+                self._brightness = int(percent_bright * 255)
             self.async_schedule_update_ha_state()
 
         if self._topic[CONF_RGB_STATE_TOPIC] is not None:
@@ -616,6 +625,27 @@ class MqttLight(MqttAvailability, MqttDiscoveryUpdate, Light):
             if self._optimistic_brightness:
                 self._brightness = kwargs[ATTR_BRIGHTNESS]
                 should_update = True
+        elif ATTR_BRIGHTNESS in kwargs and ATTR_HS_COLOR not in kwargs and\
+                self._topic[CONF_RGB_COMMAND_TOPIC] is not None:
+            rgb = color_util.color_hsv_to_RGB(
+                self._hs[0], self._hs[1], kwargs[ATTR_BRIGHTNESS] / 255 * 100)
+            tpl = self._templates[CONF_RGB_COMMAND_TEMPLATE]
+            if tpl:
+                rgb_color_str = tpl.async_render({
+                    'red': rgb[0],
+                    'green': rgb[1],
+                    'blue': rgb[2],
+                })
+            else:
+                rgb_color_str = '{},{},{}'.format(*rgb)
+
+            mqtt.async_publish(
+                self.hass, self._topic[CONF_RGB_COMMAND_TOPIC],
+                rgb_color_str, self._qos, self._retain)
+
+            if self._optimistic_brightness:
+                self._brightness = kwargs[ATTR_BRIGHTNESS]
+                should_update = True
 
         if ATTR_COLOR_TEMP in kwargs and \
            self._topic[CONF_COLOR_TEMP_COMMAND_TOPIC] is not None:
diff --git a/tests/components/light/test_mqtt.py b/tests/components/light/test_mqtt.py
index 2b23be101c7..5a768820e18 100644
--- a/tests/components/light/test_mqtt.py
+++ b/tests/components/light/test_mqtt.py
@@ -393,6 +393,41 @@ class TestLightMQTT(unittest.TestCase):
         self.assertEqual(255,
                          light_state.attributes['brightness'])
 
+    def test_brightness_from_rgb_controlling_scale(self):
+        """Test the brightness controlling scale."""
+        with assert_setup_component(1, light.DOMAIN):
+            assert setup_component(self.hass, light.DOMAIN, {
+                light.DOMAIN: {
+                    'platform': 'mqtt',
+                    'name': 'test',
+                    'state_topic': 'test_scale_rgb/status',
+                    'command_topic': 'test_scale_rgb/set',
+                    'rgb_state_topic': 'test_scale_rgb/rgb/status',
+                    'rgb_command_topic': 'test_scale_rgb/rgb/set',
+                    'qos': 0,
+                    'payload_on': 'on',
+                    'payload_off': 'off'
+                }
+            })
+
+        state = self.hass.states.get('light.test')
+        self.assertEqual(STATE_OFF, state.state)
+        self.assertIsNone(state.attributes.get('brightness'))
+        self.assertFalse(state.attributes.get(ATTR_ASSUMED_STATE))
+
+        fire_mqtt_message(self.hass, 'test_scale_rgb/status', 'on')
+        fire_mqtt_message(self.hass, 'test_scale_rgb/rgb/status', '255,0,0')
+        self.hass.block_till_done()
+
+        state = self.hass.states.get('light.test')
+        self.assertEqual(255, state.attributes.get('brightness'))
+
+        fire_mqtt_message(self.hass, 'test_scale_rgb/rgb/status', '127,0,0')
+        self.hass.block_till_done()
+
+        state = self.hass.states.get('light.test')
+        self.assertEqual(127, state.attributes.get('brightness'))
+
     def test_white_value_controlling_scale(self):
         """Test the white_value controlling scale."""
         with assert_setup_component(1, light.DOMAIN):
@@ -894,6 +929,39 @@ class TestLightMQTT(unittest.TestCase):
             mock.call('test_light/bright', 50, 0, False)
         ], any_order=True)
 
+    def test_on_command_rgb(self):
+        """Test on command in RGB brightness mode."""
+        config = {light.DOMAIN: {
+            'platform': 'mqtt',
+            'name': 'test',
+            'command_topic': 'test_light/set',
+            'rgb_command_topic': "test_light/rgb",
+        }}
+
+        with assert_setup_component(1, light.DOMAIN):
+            assert setup_component(self.hass, light.DOMAIN, config)
+
+        state = self.hass.states.get('light.test')
+        self.assertEqual(STATE_OFF, state.state)
+
+        common.turn_on(self.hass, 'light.test', brightness=127)
+        self.hass.block_till_done()
+
+        # Should get the following MQTT messages.
+        #    test_light/rgb: '127,127,127'
+        #    test_light/set: 'ON'
+        self.mock_publish.async_publish.assert_has_calls([
+            mock.call('test_light/rgb', '127,127,127', 0, False),
+            mock.call('test_light/set', 'ON', 0, False),
+        ], any_order=True)
+        self.mock_publish.async_publish.reset_mock()
+
+        common.turn_off(self.hass, 'light.test')
+        self.hass.block_till_done()
+
+        self.mock_publish.async_publish.assert_called_once_with(
+            'test_light/set', 'OFF', 0, False)
+
     def test_default_availability_payload(self):
         """Test availability by default payload with defined topic."""
         self.assertTrue(setup_component(self.hass, light.DOMAIN, {