Castelnau-Magnoac (; ) is a commune in the Hautes-Pyrénées department in south-western France.
Castelnau-Magnoac experiences an oceanic climate (Köppen climate classification Cfb). The average annual temperature is , with May being the wettest month. The average annual rainfall totals . August is typically the warmest month, with average temperatures around , while January is the coldest, averaging .
The highest temperature ever recorded in Castelnau-Magnoac was on 13 August 2003. The coldest recorded temperature was on 25 December 2001.