Mercer County is a county located in the U.S. state of New Jersey. Its county seat is Trenton 6 . Since the 2000 Census, Mercer County has been part of the New York Combined Statistical Area; before then, it was part of the Philadelphia Combined Statistical Area. However, it is actually closer to Philadelphia than New York City, and still gets most of its television and radio from Philadelphia.

Mercer County is its own Metropolitan Statistical Area, which is formally named the Trenton-Ewing MSA. 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, estimated to have risen to 367,605 as of 2006. Mercer County ranks 79th among the highest-income counties in the United States with a per capita income of $27,914.