Troncens is a commune in the Gers department of Gascony in the southwestern France.
Troncens is situated in the south of the department near the department of Hautes-Pyrénées.
The Cabournieu, right tributary of the Bouès, has its source in the town. Another tributary (left bank) of the Bouès, the Laus, runs through the town. The farms are dispersed throughout the area.