Fort de Penthièvre, Military fort in Saint-Pierre-Quiberon, France.

Fort de Penthièvre is a stone fortress positioned at the narrowest point of the Quiberon peninsula, commanding access to the strategic coastal area. The structure features ramparts and bastions typical of 18th-century coastal defense works.

Built from 1747 on plans by engineer de Marolles for Louis Jean Marie de Bourbon, Duke of Penthièvre, the redoubt closed the isthmus road. Renamed Fort Sans-Culotte, it fell on 27 June 1795 after a four-day siege by 1,500 royalists; 400 of its 700 defenders joined them. Retaken by General Hoche, it was reinforced from 1800 and completed in 1848.

The fort's ditch holds a memorial plaque to 49 Maquis members condemned and shot there between April and July 1944; a ceremony is held every 13 July. An obelisk stands in the forecourt, its granite base carved with the Breton coat of arms.

The interior is a military restricted zone and entry is not permitted; the accessible part is the fort ditch, where a memorial plaque to the executed resistance fighters stands. The 3rd Marine Infantry Regiment of Vannes has used the site for commando initiation training, with a climbing wall, since 1969. An obelisk memorial to the victims of Nazi rule was erected in the forecourt, with a granite slab bearing the Breton coat of arms at its foot.

During the French Revolution the fort was renamed Fort Sans-Culotte in 1795, a name reflecting revolutionary ideology. This change reveals how political upheaval of that era affected even military installations.