Mercer County is a county located in the U.S. state of New Jersey. Its county seat is Trenton, also the state capital, prompting its nickname The Capital County. Mercer County alone constitutes the Trenton–Princeton metropolitan statistical area and is considered part of the New York combined statistical area by the U.S. Census Bureau, but also directly borders the Philadelphia metropolitan area and is included within the Federal Communications Commission's Philadelphia Designated Media Market Area. The county is part of the Central Jersey region of the state.

New Jersey is a state situated within both the Mid-Atlantic and Northeastern regions of the United States. It is the most densely populated of all 50 U.S. states, and is situated at the center of the Northeast megalopolis. New Jersey is bordered on its north and east by New York state; on its east, southeast, and south by the Atlantic Ocean; on its west by the Delaware River and Pennsylvania; and on its southwest by Delaware Bay and Delaware. At 7,354 square miles (19,050 km2), New Jersey is the fifth-smallest state in land area, but with close to 9.3 million residents as of the 2020 United States census, its highest decennial count ever, it ranks 11th in population. The state capital is Trenton, and the state's most populous city is Newark. New Jersey is the only U.S. state in which every county is deemed urban by the U.S. Census Bureau with 13 counties included in the New York metropolitan area, seven counties in the Philadelphia metropolitan area, and Warren County part of the heavily industrialized Lehigh Valley metropolitan area.