diff --git a/homeassistant/components/prometheus/__init__.py b/homeassistant/components/prometheus/__init__.py
index 1ce16caa6e1..7beac4cc54b 100644
--- a/homeassistant/components/prometheus/__init__.py
+++ b/homeassistant/components/prometheus/__init__.py
@@ -19,6 +19,7 @@ from homeassistant.components.climate import (
 from homeassistant.components.cover import ATTR_POSITION, ATTR_TILT_POSITION
 from homeassistant.components.http import HomeAssistantView
 from homeassistant.components.humidifier import ATTR_AVAILABLE_MODES, ATTR_HUMIDITY
+from homeassistant.components.light import ATTR_BRIGHTNESS
 from homeassistant.components.sensor import SensorDeviceClass
 from homeassistant.const import (
     ATTR_BATTERY_LEVEL,
@@ -323,14 +324,14 @@ class PrometheusMetrics:
         }
 
     def _battery(self, state):
-        if "battery_level" in state.attributes:
+        if (battery_level := state.attributes.get(ATTR_BATTERY_LEVEL)) is not None:
             metric = self._metric(
                 "battery_level_percent",
                 self.prometheus_cli.Gauge,
                 "Battery level as a percentage of its capacity",
             )
             try:
-                value = float(state.attributes[ATTR_BATTERY_LEVEL])
+                value = float(battery_level)
                 metric.labels(**self._labels(state)).set(value)
             except ValueError:
                 pass
@@ -440,8 +441,9 @@ class PrometheusMetrics:
         )
 
         try:
-            if "brightness" in state.attributes and state.state == STATE_ON:
-                value = state.attributes["brightness"] / 255.0
+            brightness = state.attributes.get(ATTR_BRIGHTNESS)
+            if state.state == STATE_ON and brightness is not None:
+                value = brightness / 255.0
             else:
                 value = self.state_as_number(state)
             value = value * 100
diff --git a/tests/components/prometheus/test_init.py b/tests/components/prometheus/test_init.py
index 1e14ab848a0..af2f2ba5784 100644
--- a/tests/components/prometheus/test_init.py
+++ b/tests/components/prometheus/test_init.py
@@ -491,6 +491,12 @@ async def test_light(client, light_entities) -> None:
         'friendly_name="PC"} 70.58823529411765' in body
     )
 
+    assert (
+        'light_brightness_percent{domain="light",'
+        'entity="light.hallway",'
+        'friendly_name="Hallway"} 100.0' in body
+    )
+
 
 @pytest.mark.parametrize("namespace", [""])
 async def test_lock(client, lock_entities) -> None:
@@ -1557,6 +1563,19 @@ async def light_fixture(
     data["light_4"] = light_4
     data["light_4_attributes"] = light_4_attributes
 
+    light_5 = entity_registry.async_get_or_create(
+        domain=light.DOMAIN,
+        platform="test",
+        unique_id="light_5",
+        suggested_object_id="hallway",
+        original_name="Hallway",
+    )
+    # Light is on, but brightness is unset; expect metrics to report
+    # brightness of 100%.
+    light_5_attributes = {light.ATTR_BRIGHTNESS: None}
+    set_state_with_entry(hass, light_5, STATE_ON, light_5_attributes)
+    data["light_5"] = light_5
+    data["light_5_attributes"] = light_5_attributes
     await hass.async_block_till_done()
     return data