Onset is a census-designated place (CDP) in the town of Wareham in Massachusetts. The population was 1,292 at the 2000 census.
Onset is located at 41°44′47″N 70°39′48″W / 41.74639°N 70.66333°W / 41.74639; -70.66333 (41.746424, -70.663251).
According to the United States Census Bureau, the CDP has a total area of 3.4 km² (1.3 mi²). 2.8 km² (1.1 mi²) of it is land and 0.6 km² (0.2 mi²) of it (16.92%) is water.