Third class municipality in the province of Marinduque, Philippines. According to the 2010 census, it has a population of 33,402 people. The municipality is bounded by the provincial capital, Boac, to the north and east, by Buenavista to the southeast and by the Sibuyan Sea to the south and west. It is the second-oldest municipality in Marinduque, after Boac. Residents of Gasan are called Gaseños.
The Tres Reyes Islands off the coast of Marinduque are under the jurisdiction of Gasan, part of the municipality's Barangay Pinggan.
