Manchac Swamp Bridge, Road bridge in St. John the Baptist Parish, Louisiana, US.

The Manchac Swamp Bridge is a twin concrete trestle bridge near Manchac, Louisiana, carrying Interstate 55 and U.S. Route 51 over the Manchac Swamp. Its deck is a flat horizontal girder resting on evenly spaced pairs of plain gray cylindrical columns joined by cap beams, with no signs or markings on the concrete. The structure runs 22.80 miles (36.69 km) and recedes straight into the distance.

The bridge was built in the late 1970s to create a faster route through the Louisiana swamps and officially opened to traffic in 1979. Its construction required deep foundation piles because the ground consists of soft peat and mud that could not support conventional foundations.

Manchac is an Native American word meaning, loosely translated, "back entrance": through Pass Manchac, boats reach Lake Maurepas from Lake Pontchartrain and then the Mississippi via Bayou Manchac, a back way from the Gulf of Mexico to the river that avoids its main mouth.

The bridge carries two major highways across the swamp and stays open to all types of vehicles throughout the week. In fog or heavy rain it is wise to drive slower and keep your lights on, because visibility can drop sharply along the long straight stretch.

The bridge's piles were driven 250 feet (76 m) beneath the swamp to carry the deck, and construction cost about $7 million per mile. It is the longest bridge on the Interstate Highway System, and some claim it is the longest toll-free road bridge in the world. The name Manchac comes from a Native American word meaning roughly "back entrance", for the water route from Lake Pontchartrain through Pass Manchac to Lake Maurepas and the Mississippi.