Mont-de-Marsan is a commune and capital of the Landes department, Nouvelle-Aquitaine, southwestern France.

Landes is a department in the Nouvelle-Aquitaine region, Southwestern France, with a long coastline on the Atlantic Ocean to the west. It borders Gers to the east, Pyrénées-Atlantiques, to the south, Lot-et-Garonne to the north-east, and Gironde to the north. It also borders the Atlantic Ocean to the west. Located on the Atlantic coast, it had a population of 413,690 as of 2019. Its prefecture is Mont-de-Marsan.