Castelnau-Montratier (; Languedocien: Castèlnau de Montratièr) is a former commune in the Lot department in south-western France. On 1 January 2017, it was merged into the new commune Castelnau-Montratier-Sainte-Alauzie, which was renamed Castelnau-Montratier effective 2024.
The Barguelonne flows southwestward through the northern part of the commune.