Route 18 is a 42.8-mile-long (68.9 km) state highway in the central part of the US state of New Jersey. It begins at an intersection with Route 138 in Wall Township, Monmouth County and ends at Interstate 287 (I-287) in Piscataway Township, Middlesex County. Route 18 is a major route through central New Jersey that connects the Jersey Shore to the Raritan Valley region, connecting the county seats of Monmouth (Freehold) and Middlesex respectively. The route runs through Ocean Township, Marlboro Township, East Brunswick, and is the main thoroughfare for Rutgers University. Much of the route is a freeway. The remainder of the route is an arterial road with traffic lights in the East Brunswick and Old Bridge areas, and a boulevard in the remainder of Piscataway. Route 18 was designated in 1939 as a proposed freeway from Old Bridge to Eatontown. The section west of Old Bridge was formerly designated as part Route S28, a prefixed spur of State Highway Route 28 from Middlesex to Matawan. The designation, assigned in the 1927 renumbering, remained until a second renumbering in 1953. At that point, Route S28 was redesignated as Route 18, though the section from Old Bridge to Matawan was signed as TEMP 18, as this section would be decommissioned when the Route 18 freeway was built.

Middlesex County is located in the north-central part of the U.S. state of New Jersey, extending inland from the Raritan Valley region to the northern portion of the Jersey Shore. As of the 2020 United States census, the county was the state's third-most populous county with a population of 863,162, its highest decennial count ever and an increase of 53,304 (+6.6%) from the 2010 census count of 809,858, which in turn reflected an increase of 59,696 (8.0%) from the 750,162 counted in the 2000 census. Middlesex is part of the New York metropolitan area. Many communities within the county serve as commuter towns to and from New York City and other points north. The county is part of the Central Jersey region of the state.