diff --git a/homeassistant/components/generic_thermostat/climate.py b/homeassistant/components/generic_thermostat/climate.py
index ef3cf11fa1c..e83852d122f 100644
--- a/homeassistant/components/generic_thermostat/climate.py
+++ b/homeassistant/components/generic_thermostat/climate.py
@@ -1,6 +1,7 @@
 """Adds support for generic thermostat units."""
 import asyncio
 import logging
+import math
 
 import voluptuous as vol
 
@@ -419,7 +420,10 @@ class GenericThermostat(ClimateEntity, RestoreEntity):
     def _async_update_temp(self, state):
         """Update thermostat with latest state from sensor."""
         try:
-            self._cur_temp = float(state.state)
+            cur_temp = float(state.state)
+            if math.isnan(cur_temp) or math.isinf(cur_temp):
+                raise ValueError(f"Sensor has illegal state {state.state}")
+            self._cur_temp = cur_temp
         except ValueError as ex:
             _LOGGER.error("Unable to update from sensor: %s", ex)
 
diff --git a/tests/components/generic_thermostat/test_climate.py b/tests/components/generic_thermostat/test_climate.py
index c2c1435464e..f5a27ac8b97 100644
--- a/tests/components/generic_thermostat/test_climate.py
+++ b/tests/components/generic_thermostat/test_climate.py
@@ -331,9 +331,18 @@ async def test_sensor_bad_value(hass, setup_comp_2):
 
     _setup_sensor(hass, None)
     await hass.async_block_till_done()
-
     state = hass.states.get(ENTITY)
-    assert temp == state.attributes.get("current_temperature")
+    assert state.attributes.get("current_temperature") == temp
+
+    _setup_sensor(hass, "inf")
+    await hass.async_block_till_done()
+    state = hass.states.get(ENTITY)
+    assert state.attributes.get("current_temperature") == temp
+
+    _setup_sensor(hass, "nan")
+    await hass.async_block_till_done()
+    state = hass.states.get(ENTITY)
+    assert state.attributes.get("current_temperature") == temp
 
 
 async def test_sensor_unknown(hass):