The Treaty of Bassein was signed by Sultan Bahadur of Gujarat and the Kingdom of Portugal on 23 December 1534 while on board the galleon São Mateus. Based on the terms of the agreement, the Portuguese Empire gained control of the city of Bassein, as well as its territories, islands, and seas. The Mumbai islands which fell under Portuguese control included Colaba, Old Woman's Island, Mumbai, Mazagaon, Worli, Matunga, and Mahim. Salsette, Daman and Diu, Thane, Kalyan, and Chaul were other territories controlled and settled by the Portuguese.
At the time, the cession of Mumbai (or Bombay) was of minor importance, but it gained a crucial importance when the place passed from the Portuguese to the English in 1661 as part of the dowry of Catherine of Braganza, and became a major trade center, making this the treaty's most important long-term result.