Merrimack County is a county in the U.S. state of New Hampshire which has two cities and twenty-five towns. In 2006 the population was 148,085. Its county seat is Concord. The center of population of New Hampshire is located in Merrimack County, in the town of Pembroke.
Merrimack County was organized at Concord in 1823, and is named for the Merrimack River. It was formed by removing towns from northern Hillsborough and Rockingham Counties. In 1841 its original scope was somewhat reduced by the creation of Belknap County.
The county has a total area of 956 square miles (2,476 km2), of which 934 sq mi (2,419 km2) is land and 22 sq mi (57 km2) (2.31%) is water.