Route 12 is a state highway located in Hunterdon County, New Jersey, United States. It runs 11.69 mi (18.81 km) from the Uhlerstown-Frenchtown Bridge at the Delaware River border with Pennsylvania in Frenchtown east to an intersection with U.S. Route 202 and Route 31 at the Flemington Circle in Flemington. The route is mostly a two-lane undivided road that passes through rural areas of woodland and farmland. It intersects Route 29 and County Route 513 in Frenchtown, County Route 519 in Kingwood Township, County Route 579 on the border of Delaware Township and Raritan Township, and County Route 523 in Raritan Township.

The route was designated in 1927 to run from Frenchtown to Raritan, Somerset County, running along its current alignment to Flemington and following present-day U.S. Route 202 between Flemington and Raritan. By the 1940s, U.S. Route 202 and Route 29 replaced Route 12 east of Flemington, and the route was officially designated to end in Flemington in 1953. There are currently plans to modify or eliminate three traffic circles along the route in Flemington to ease traffic congestion.

Route 12 begins at the Uhlerstown-Frenchtown Bridge over the Delaware River in Frenchtown at the Pennsylvania/New Jersey border, where it connects to Pennsylvania Route 32. The bridge has a 15 mph (24 km/h) speed limit and is maintained by the Delaware River Joint Toll Bridge Commission. The route continues through the downtown area of Frenchtown as 25 mph (40 km/h) county-maintained two-lane undivided Bridge Street. Here, it intersects the northern terminus of Route 29 (Trenton Road) before turning north and intersecting the southern terminus of County Route 513 (Race Street). Route 12 heads east on 30 mph (48 km/h) Kingwood Avenue, with the speed limit increasing to 40 mph (64 km/h) as it passes through residential areas. The route crosses into Kingwood Township, where it becomes a 50 mph (80 km/h) road maintained by the New Jersey Department of Transportation with two eastbound lanes and one westbound lane. It heads through a mix of woods and farms with intermittent residences, narrowing to one lane in each direction before crossing County Route 519 (Kingwood Road) in Baptistown.

Route 12 continues southeast through agricultural areas, crossing the Lockatong Creek before intersecting County Route 615 (Pittstown Road), where it becomes the Samuel Hill Highway. A short distance past this intersection, the route briefly passes through the southern corner of Franklin Township before entering Delaware Township. Here, the road crosses Wickecheoke Creek before coming to an intersection with County Route 579 (Croton Road/Easton-Trenton Turnpike). Past this intersection, Route 12 enters Raritan Township, where it continues through rural areas of woodland and farmland, with some residences. The residential development along the road increases before it reaches the Dvoor's Circle (a traffic circle) with County Route 523 (Mine Street/Flemington Road), with the speed limit dropping to 45 mph (72 km/h) and then to 40 mph (64 km/h). Past the traffic circle, the route crosses a small stream and enters Flemington. Here, Route 12 passes businesses before crossing the Black River & Western Railroad and coming to the Main Street Circle, where it intersects County Route 611 (Main Street) and Reaville Avenue. From here, the route continues east a short distance as a four-lane divided highway with a 30 mph (48 km/h) speed limit before ending at U.S. Route 202 and Route 31 at the Flemington Circle.

Route 12 was legislated as a new route in the 1927 New Jersey state highway renumbering that was to run from Frenchtown east to Route 28 in Raritan, Somerset County, running along its current alignment to Flemington and following present-day U.S. Route 202 to Raritan. By the 1940s, U.S. Route 202 and Route 29 were designated along the alignment of Route 12 between Flemington and Raritan, cutting the route’s eastern terminus back to Flemington. In the 1953 New Jersey state highway renumbering, Route 12 was officially legislated to run from Frenchtown to U.S. Route 202 and Route 69 (now Route 31) in Flemington. There are currently plans to modify or eliminate the three traffic circles along Route 12 in Flemington in order to ease traffic congestion along the route. The Main Street Circle is planned to become a roundabout while plans are still being thought out for the other two traffic circles.