Castelvieilh (; ) is a commune in the Hautes-Pyrénées department in south-western France.
Castelvieilh has an oceanic climate (Köppen climate classification Cfb) closely bordering on a humid subtropical climate (Cfa). The average annual temperature in Castelvieilh is . The average annual rainfall is with May as the wettest month. The temperatures are highest on average in August, at around , and lowest in January, at around . The highest temperature ever recorded in Castelvieilh was on 4 August 2003; the coldest temperature ever recorded was on 8 February 2012.