Petersham is a town in Worcester County, Massachusetts, United States. The population was 1,194 at the 2020 census. Petersham is home to a considerable amount of conservation land, including the Quabbin Reservation, Harvard Forest, the Swift River Reservation, and Federated Women's Club State Forest.

Worcester County is a county in the U.S. state of Massachusetts. At the 2020 census, the population was 862,111, making it the second-most populous county in Massachusetts. It is also the largest county in Massachusetts by geographic area. The largest city and traditional shire town is Worcester. Worcester County is part of the Worcester, MA–CT metropolitan statistical area and the Boston-Worcester-Providence combined statistical area.