Merrimac is a small town in Essex County, Massachusetts, United States, and on the southeastern border of New Hampshire, approximately 34 miles (55 km) northeast of Boston and 10 miles (16 km) west of the Atlantic Ocean. It was incorporated on April 11, 1876. It is situated along the north bank of the Merrimack River in the Merrimack Valley. The population was 6,338 at the 2010 census. Historically a manufacturing center, it has long since become a largely residential community. It is part of the Greater Boston metropolitan area.

Settled by the English in 1638 as a part of Salisbury and later as a part of Amesbury around the village of Merrimacport, it was known throughout the seventeenth and eighteenth centuries as an agricultural and fishing community, with a small amount of shipbuilding. When Amesbury separated from Salisbury in 1666, Merrimac was referred to as the West Parish of Amesbury, or simply West Amesbury, although it was unincorporated. When a border dispute between the Massachusetts and New Hampshire colonies was settled in 1741, the new border sliced off the parts of Amesbury that were further from the Merrimack River, with the area then associated with West Amesbury becoming the "new town" of Newton, New Hampshire.

In the nineteenth century, benefiting from a manufacturing boom following the establishment of some of the first planned industrial cities in the United States, nearby Lawrence and Lowell, Merrimac came to be known worldwide for its horse-drawn carriage industry. During this period, the town proper of Merrimac, centered around Merrimac Square, expanded separately from the village of Merrimacport. In 1876, Merrimac, including Merrimacport, separated from Amesbury and officially incorporated itself as a town. It is believed that the town, as well as the river that runs along its southern border, are both named for the American Indian tribe that occupied the region. "Merrimac" (or Merrimack) means "swift water place" in the language of this tribe. This town center consists of the typical brick buildings and Victorian architecture of the late nineteenth century, and it is surrounded by much of the town's population. Interstate 495 now divides Merrimacport from Merrimac. At the beginning of the twentieth century, as with the rest of the New England, it went through a period of deindustrialization as the region's industry relocated to the Midwest. The communities of the Merrimack Valley, including Merrimac, were particularly affected by this long period of economic decline and have never fully recovered.

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