Fort Diamant, Military fort in Remire-Montjoly, French Guiana.

Fort Diamant is a military fortress ruin along the coast with thick basalt and brick walls forming a V-shaped redan and defensive positions. The structure is divided into three distinct sections containing former military quarters and remains of cannon emplacements.

A wooden fort was built around 1650 on the site to protect the French from other Europeans and Amerindian raids. Portuguese forces occupied French Guiana in 1809 and held the position until their withdrawal on 21 November 1817. The present fort was begun in 1840 and finished in 1849, the delay caused by late deliveries of materials. It was regarrisoned during the Second World War.

The fort's name comes from Diamond Rock, the islet it faces at the mouth of the Mahury. Its nine-cell casemated traverse was closed by thick doors of wacapou, a very hard local wood, reinforced with iron. Since 1985 the site has been restored in stages and now belongs to the Département de la Guyane, which opens it occasionally to school groups.

The ruins can be accessed along the main beach road and are easy to explore on foot. Visit during low tide or calm weather for the safest and most comfortable experience on the rocky coastal terrain.

A deep moat filled with water once surrounded the entire structure, making it a formidable defensive position. This water barrier demonstrates how carefully the builders chose every detail to control access to the river entrance.