Onset is a census-designated place (CDP) in the town of Wareham, Massachusetts, United States. The population was 1,573 at the 2010 census. According to the United States Census Bureau, the CDP has a total area of 3.4 km² (1.3 mi²), of which 2.8 km² (1.1 mi²) is land and 0.6 km² (0.2 mi²) (16.92%) is water.
Onset was developed in the 1880s as a summer camp meeting for Spiritualists. Many of the existing cottages in Onset were built as second homes for individuals from Boston, Taunton, Brockton and other northeastern cities who gathered to hear mediums communicate with the dead. While it was run by the Spiritualists, the village was known as Onset Bay Grove.