Merrimac is a small town in Essex County, Massachusetts, United States, located 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 6,723 at the 2020 census. Historically a mill town, it has long since become a largely residential community. It is part of the Greater Boston metropolitan area.

Essex County is a county in the northeastern part of the U.S. state of Massachusetts. At the 2020 census, the total population was 809,829, making it the third-most populous county in the state, and the seventy-eighth-most populous in the country. It is part of the Greater Boston area. The largest city in Essex County is Lynn. The county was named after the English county of Essex. It has two traditional county seats: Salem and Lawrence. Prior to the dissolution of the county government in 1999, Salem had jurisdiction over the Southern Essex District, and Lawrence had jurisdiction over the Northern Essex District, but currently these cities do not function as seats of government. However, the county and the districts remain as administrative regions recognized by various governmental agencies, which gathered vital statistics or disposed of judicial case loads under these geographic subdivisions, and are required to keep the records based on them. The county has been designated the Essex National Heritage Area by the National Park Service.