city, seat (1827) of Waldo county, southern Maine, U.S., on the Passagassawakeag River where it empties into Penobscot Bay on the Atlantic coast opposite Castine, 34 miles (55 km) south-southwest of Bangor. Settled in 1770 and named for Belfast, Ireland, it soon developed as a seaport and became a port of entry. Distinguished architecture of the sailing era remains. Its harbour is now used mainly by tugboats and pleasure craft. Tourism, the sardine industry, and light manufacturing are its main economic assets. The Penobscot Marine Museum is 4 miles (6 km) to the northeast in Searsport. Fort Knox (built in 1844) and Lake St. George state parks are nearby. Inc. town, 1773; city, 1853. Pop. (1990) 6,355; (2000) 6,381.

The site of Belfast was occupied during both the Stone and Bronze ages, and the remains of Iron Age forts are discernible on the slopes near the city centre. A castle, probably built there about 1177 by John de Courci, the Norman conqueror of Ulster, seems to have survived until the beginning of the 17th century. The city's name is derived from the Gaelic Béal Feirste (Mouth of the Sandbank 【or Crossing of the River】). Belfast's modern history began in 1611 when Baron Arthur Chichester (Chichester, Arthur Chichester, Baron) built a new castle there. He did much to encourage the growth of the town, which received a charter of incorporation in 1613. Belfast survived the Irish insurrection of 1641, and by 1685 it had a population of about 2,000, largely engaged in brick, rope, net, and sailcloth making. By the late 1730s the castle had been destroyed, but Belfast was beginning to acquire economic importance, superseding both Lisburn as the chief bridge town and Carrickfergus as a port. It became the market centre of the Ulster linen industry, developed by French Huguenot refugees under the patronage of William III of Great Britain at the end of the 17th century. Attempts to establish a cotton industry there were short-lived, but following mechanization of the spinning and weaving of linen, Belfast became one of the greatest linen centres in the world. By the 17th century, the town was a busy port with small shipbuilding interests, which became firmly established after William Ritchie founded a shipyard (1791) and a graving (dry) dock (1796). Since the Industrial Revolution, the chief shipbuilding firm has been Harland and Wolff (builders of the ill-fated “Titanic”). The city was severely damaged by air raids in 1941. Beginning in the 1970s, Belfast's traditional manufacturing specialties, linen and shipbuilding, began a long decline. These sectors are now overshadowed by service activities, food processing, and machinery manufacture.