Winchendon is a town in Worcester County, Massachusetts, United States. The population was 10,300 at the 2010 census. The town includes the villages of Waterville and Winchendon Springs. A census-designated place, also named Winchendon, is defined within the town for statistical purposes. The Winchendon State Forest, a 174.5 acres parcel, is located within the township as is Otter River State Forest; both recreational areas are managed by the Massachusetts Department of Conservation and Recreation.