Merrimack County is a county in the U.S. state of New Hampshire. As of the 2020 census, the population was 153,808,^[1] making it the third-most populous county in New Hampshire. Its county seat is Concord,^[2] the capital of New Hampshire. The county was founded in 1823 from parts of Hillsborough and Rockingham counties, and is named for the Merrimack River.^[3]

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