Sindhudurg Fort, Sea fort in Sindhudurg district, India.

Sindhudurg Fort sits on an island in the Arabian Sea and covers about 19 hectares with thick stone walls all around. The walls rise roughly 30 feet (9 meters) high and measure around 12 feet (4 meters) wide.

Shivaji chose Khurte island near Malvan for a sea fort and laid its foundation stone on 25 November 1664. Hiroji Indulkar supervised the work with help from Portuguese engineers from Goa, using 200 Vaddera builders; the fort was finished in 1667. Its purpose was to check European trading companies and the Siddhis of Janjira. The English later took the fort and partly demolished it.

The name means fort in the ocean and describes the location on the small island precisely. Temples and shrines inside the walls are still visited today by pilgrims and travelers.

Boats leave from Malvan harbour for the roughly half-mile crossing to Khurte island and land near the fort. The main gate faces east and is hidden from the sea; from the landing a north-facing gap leads to a door of umbar wood. The fort is closed to tourists during the monsoon because of high tides.

The fort's rampart once carried a saffron Maratha flag on a pole 228 feet tall, visible far out at sea until 1812. Fishermen used the flag as a bearing to keep clear of the rock. The flagstaff's height is recorded in Marathi accounts of the fort.