Kolaba Fort (sometimes Colaba Fort or Alibaug Fort) is an old fortified maritime base in Alibaug, Konkan, India. It is situated in the sea at a distance of 1–2 km from the shores of Alibag, 35 km south of Mumbai, in the Konkan region of Maharashtra, India. It is a popular tourist destination and a protected monument.
The first mention of Kolaba fort is when it was chosen by Chatrapati Shivaji Maharaj to be fortified after the whole of South Konkan became free. The work of constructing the fort started in 19 March 1680. In 1662, he strengthened and fortified Kolaba fort to make it one of his chief naval stations. The command of the fort was given to Darya Sarang and Mainak Bhandari under whom Kolaba Fort became the centre of the Maratha attacks on British ships. Kolaba Fort was captured by Chatrapati Shivaji .
The fort was completed by Chatrapati Sambhaji Raje after the death of Chatrapati Shivaji Maharaj in June 1681. In 1713 under a treaty with Peshwa Balaji Vishwanath, Kolaba along with several other forts was given over to Sarkhel Kanhoji Angre. He used it as his main base from which to launch raids on British ships. In 17 November 1721, the British, incensed at Angre’s activities, joined the Portuguese in an expedition against Kolaba. A Portuguese land force of 6000 and three English ships of the line under Commodore Mathews co-operated but the attempt failed. The British blamed the failure on the “cowardice of the Portuguese”. About this time Kolaba is described by Hamilton as a fort built on a rock, a little way from the mainland and at high water an island. On 4 July 1729, Kanhoji Raje Angre died on the Kolaba Fort. In 1729 many buildings were destroyed due to a major fire incident near the Pinjara Bastion. In 1787 another major fire incident took place in which the Angre Wada was destroyed. In 1842 the British sold the wooden structures in the fort by auction and used the stones for the construction of Alibag water works.