The Treaty of Bassein was signed by Sultan of Guzerat and the Kingdom of Portugal on 23 December 1534, while on board the galleon São Mateus. Based on the terms of the treaty, the Portuguese Empire gained control of the city centre of Baçaim now Bassein (Vasai), as well as the neighbouring subjected tracts of land and stretches of sea. The Bombay Islands (Ilhas de Bombaim) which were then ruled from Portuguese Goa included: Colabá Grande, Colabá Pequeno, Bom Bahia (Bombay), Mazagaon, Vorlim, Matungá, and Mahim. Salsette, Damão é Dio, Taana, Kalyan & Chaul were among the town centres controlled and settled by the Portuguese.
| Signed | 23 December 1534 |
|---|---|
| Location | At sea, aboard the Portuguese galleon São Mateus |
| Mediators | An envoy from the Ottoman Empire |
| Negotiators | |
| Signatories |
.svg){kind=small}
At the time, the cession of 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.