U.S. Route 51 in Louisiana in the context of Lake Maurepas

The Manchac Swamp Bridge is a twin concrete trestle bridge near Manchac in the U.S. state of Louisiana. It carries Interstate 55 and U.S. Route 51 over the Manchac Swamp in Louisiana and represents a third of the highway's approximately 66 miles (106 km) in Louisiana.

With a total length of 22.80 miles (36.69 km), it is one of the longest bridges in the world over water, and is the longest bridge on the Interstate Highway System, and some claim it is the longest toll-free road bridge in the world.

Interstate 55 (I-55) is a part of the Interstate Highway System that spans 964.25 miles (1,551.81 km) from LaPlace, Louisiana, to Chicago, Illinois. Within the state of Louisiana, the highway travels 66 miles (106 km) from the national southern terminus at I-10 in LaPlace to the Mississippi state line north of Kentwood.

The route is located in the southeastern portion of Louisiana and parallels the older U.S. Highway 51 (US 51) corridor. While passing through the city of Hammond, I-55 intersects two of the state's major east–west routes, I-12 and US 190. It also serves the smaller city of Ponchatoula, as well as the towns of Amite City and Kentwood.