The Fort de Bertheaume is a fort in Plougonvelin, in the Department of Finistère, France. It is located on a tidal island that nowadays connects to the mainland via a footbridge. The fort sits well above sea level, and its steep cliffs have rendered it easily defended for centuries. Since the 17th Century the fort's role has been to monitor the Goulet de Brest, the straits of Brest.