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.

Petersham was first settled by Europeans in 1733 and was officially incorporated on April 20, 1754. On February 4, 1787, it was the site of the second battle of Shays’ Rebellion. The town is noted for its common, part of the Petersham Common Historic District. About 45 buildings are listed on the National Register of Historic Places. The Country Store, an 1842 Greek Revival structure that has housed a general store on its main floor since its opening, sits just to the East of the common.

The town’s lands were expanded greatly by the building of the Quabbin Reservoir in 1938. When the towns of the Swift River Valley were disincorporated, Petersham and neighboring New Salem benefited the most, with Petersham receiving all of the former town of Dana, much of the town of Greenwich, and a small portion of the former town of Prescott east of the Middle Branch of the Swift River. Its modern southwestern borders lie along the former East and Middle branches of the Swift River, and includes lands that were once part of Hampshire County.

A memorial was erected in the town in 1927 by the New England Society of Brooklyn, New York. The memorial commemorates General Benjamin Lincoln, who raised 3,000 troops and routed the rebellion on February 4, 1787. It ends with the line, “Obedience to the law is true liberty.”

According to the United States Census Bureau, the town has a total area of 68.3 square miles (177 km), of which 54.2 square miles (140 km²) is land and 14.1 square miles (37 km) (20.60%) is water.^{[citation needed]} By virtue of the lands it gained in the construction of the Quabbin Reservoir, Petersham is the largest town by land area in Worcester County, and the fifth-largest town of the 351 cities and towns in the Commonwealth. It is the largest town by area in Central and Western Massachusetts, with the four largest towns being in southeastern Massachusetts (Plymouth, Middleborough, Dartmouth and Barnstable).^{[citation needed]}

Much of the town’s land is protected as part of the Quabbin Reservation, a protected area surrounding the Quabbin Reservoir. The town’s borders extend well into the eastern branch of the reservoir, and includes the lands around what were once Mount Pomeroy, Mount Zion (the largest such “island” in the reservoir), and several other former hills. The lands of the Reservation are only accessible on foot, and include the only disincorporated areas which are accessible to the public, at the former Dana Common, a 1.5 miles (2.4 km) walk from Gate 40. Most of the town still drains into the Quabbin, along brooks that once met the Swift River. The town has several other hills that are part of the larger Worcester Hills region, including Whitney Hill and Camels Hump Hill, at 1,044 feet (318 m) the highest point in town. Several other parts of the town are also protected, including the Federated Women’s Club State Forest, the Petersham State Forest, Harvard state Forest, the Popple Camp Wildlife Management Area, and the Phillipston Wildlife Management Area, the latter three extending into neighboring towns. Most of the town is rural in nature, with the largest population located near the town common, in the village of Petersham.^{[citation needed]}