county, southwestern Massachusetts, U.S., bordered by Connecticut to the south. The county's terrain is characterized by mountains in the west and by ridges and valleys in the east, bisected north-south by the Connecticut River. Other watercourses include the Chicopee and Westfield rivers and the Cobble Mountain Reservoir. Chester-Blandford, Granville, and Tolland state forests are found in the west, while Brimfield State Forest is in the east.

The county, created in 1812, is named for John Hampden (Hampden, John) (1594–1643), the English Parliamentary leader who opposed King Charles I. The county seat is Springfield; factories there manufactured firearms (notably the Springfield and M-1 rifles) used in the U.S. War of Independence, the American Civil War, and World Wars I and II. Other cities are Chicopee, Holyoke, and Westfield, the home of Westfield State College (founded 1839). The sports of basketball and volleyball originated in the county in the 1890s (in Springfield and Holyoke, respectively). Westover Air Force Base (opened 1939) was heavily used in World War II.

Principal industries are health care services and the manufacture of computers, sporting goods, small arms, and dairy, paper, and plastic products. Area 619 square miles (1,602 square km). Pop. (2000) 456,228; (2007 est.) 457,908.