Fort Lachaux, Military fortress in Grand-Charmont, France

Fort Lachaux is a Séré de Rivières fortress on the Lachaux hill at 410 metres, on the border of Montbéliard and Grand-Charmont. Only the entrance, the casernement and part of the wall with its double caponier and ditch remain; the rest was demolished in the 1950s for the Peugeot factory. The entrance wall bears the inscription "FORT LACH AUX" above a double metal door.

The fortress was built between 1876 and 1878 by engineer Adrien Hallier as part of the Lomont defensive system. It was later integrated into the Belfort fortification network, becoming part of the region's military defense strategy.

The fort's name briefly changed to Fort Razout in 1887 under a decree by War Minister Georges Boulanger, then reverted that same year. Today the site hosts a educational farm, a radio-controlled car track and an escape game, while the casernement serves as a shooting range for the Police nationale.

Access to the fort is only possible with permission, as the Police nationale runs a shooting range in the remaining barracks. The site lies on the Lachaux hilltop at 410 metres, on the boundary of Montbéliard and Grand-Charmont, along the Rue de Fort Lachaux; it belongs to Pays de Montbéliard Agglomération.

The fortress contained an optical telegraph system that allowed communication with neighboring forts and improved military coordination. This technology was an early example of long-distance communication techniques used during the period.