List of Massachusetts counties:
- Barnstable County: one of three original counties created in the Plymouth Colony in 1685. County seat: Barnstable.
- Berkshire County: formed in 1761 from part of Hampshire County. County seat: Pittsfield.
- Bristol County: one of three original counties created in the Plymouth Colony in 1685. County seats: Taunton and Fall River.
- Dukes County: formed in 1695 from Martha's Vineyard and the Elizabeth Islands, which had been part of Dukes County, New York until Massachusetts gained it in 1691. County seat: Edgartown.
- Essex County: one of four original counties created in Massachusetts in 1643. County seats: Salem and Lawrence.
- Franklin County: formed in 1811 from part of Hampshire County. County seat: Greenfield.
- Hampden County: formed in 1812 from part of Hampshire County. County seat: Springfield.
- Hampshire County: formed in 1662 from the western part of Massachusetts. County seat: Northampton.
- Middlesex County: one of four original counties created in Massachusetts in 1643. County seats: Cambridge and Lowell.
- Nantucket County: formed in 1695 from Nantucket Island which had been part of Dukes County, New York until Massachusetts gained it in 1691. County seat: Nantucket.
- Norfolk County*: formed in 1793 from part of Suffolk County. County seat: Dedham.
- Plymouth County: one of three original counties created in the Plymouth Colony in 1685. County seats: Plymouth and Brockton.
- Suffolk County: one of four original counties created in Massachusetts in 1643. County seat: Boston.
- Worcester County: formed in 1731 from parts of Hampshire County, Middlesex County, and Suffolk County. County seats: Worcester and Fitchburg.