Mercer County is a county located in the U.S. state of New Jersey. Its county seat is Trenton6. Mercer County is in the New York metropolitan area. The county is named for Continental Army General Hugh Mercer, who died at the Battle of Princeton in 1777. As of the United States 2000 Census, the population was 350,761.

As of the census˛ of 2000, there were 350,761 people, 125,807 households, and 86,303 families residing in the county. The population density was 599/km˛ (1,552/mi˛). There were 133,280 housing units at an average density of 228/km˛ (590/mi˛). The racial makeup of the county was 68.48% White, 19.81% Black or African American, 0.20% Native American, 4.94% Asian, 0.10% Pacific Islander, 4.29% from other races, and 2.17% from two or more races. 9.66% of the population were Hispanic or Latino of any race.