Route 31 is a state highway in New Jersey, United States. It runs from U.S. Route 1 Business (US 1 Bus.) /US 206 in Trenton, Mercer County north to an intersection with US 46 in Buttzville in White Township, Warren County. Along the way, Route 31 heads through the communities of Flemington, Clinton and Washington. Most of the highway is state-maintained; however, the section within the city limits of Trenton is maintained by the city. Much of Route 31 is a two-lane highway that passes through farmland, woodland, and mountainous areas. Two portions of the route—from Trenton to Pennington and from Ringoes to Clinton—consist of more development.