Salsette Island is an island in Maharashtra state on India's west coast. The metropolis of Mumbai (formerly Bombay), the city of Thane and Mira-Bhayandar lie on this island, making it a very populous island and one of the most densely populated island in the world. It has about 15.1 million inhabitants living on an area of about 619 km².

Salsette is bounded on the north by Vasai Creek, on the northeast by the Ulhas River, on the east by Thane Creek and Bombay Harbour, and on the south and west by the Arabian Sea. The original seven islands of Bombay, which were merged by land reclamation during the 19th and early 20th centuries to form the city of Mumbai is now practically a southward protruding peninsula of the much larger Salsette Island. The island of Trombay that lay to the southeast of Salsette is today part of Salsette as much of the intervening swamps have been reclaimed. The island contains Borivali National Park, also known as Sanjay Gandhi National Park. The city of Thane lies at the island's north-eastern corner, on Thane Creek. Politically, the Mumbai City district covers the peninsula south of Mahim and Sion while most of the original island constitutes the Mumbai Suburban District. The northern portion of the island lies within Thane District, which extends across Vasai and Thane creeks onto the mainland.