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.

Route 138 is a state highway located entirely in Monmouth County, New Jersey that extends for 3.52 miles (5.66 km). The route's western terminus is at the eastern end of Interstate 195 (I-195) at the interchange with Route 34 in Wall Township. Route 138's eastern terminus is at Route 35 in Wall Township. The highway is also meant to be an evacuation route in case of a disaster, where the eastbound lanes would be reversed all the way to the New Jersey Turnpike.