Chelmsford, town (township), Middlesex county, northeastern Massachusetts, U.S. It lies 24 miles (39 km) north of Boston; the city of Lowell is adjacent to the northeast. Settled in 1633, it was named for Chelmsford, England, and incorporated in 1655. An iron foundry using local bog ore was built there in 1656. During the early 19th century, the production of lumber, textiles, and granite was important. Services, trade, and the production of electronics, computer equipment, and other light manufactures are now central to the economy.

In 1657 John Fiske, an early pastor of the Chelmsford Unitarian Church, compiled the Chelmsford Catechism, the only extant copy of which is in the New York Public Library. Joseph Spalding of Chelmsford is said to have fired the first shot in the Battle of Bunker Hill (1775). Warren H. Manning State Forest is nearby. Area 23 square miles (60 square km). Pop. (2000) 33,858; (2010) 33,802.