The Smithtown Trestle (also known as the Smithtown Viaduct and the Nissequogue River Trestle) is a railroad trestle carrying the Port Jefferson Branch of the Long Island Rail Road over the Nissequogue River in Smithtown, Suffolk County, New York.

Standing at a height of approximately 50 feet (15 meters) and measuring 484 feet (148 meters) in length, the Smithtown Trestle carries the LIRR's Port Jefferson Branch over the valley below, Jericho Turnpike (NY 25), and the Nissequogue River.

[3] The original Smithtown Trestle opened in 1873, as part of the Smithtown and Port Jefferson Railroad – an LIRR subsidiary chartered in 1870 to construct an extension of today's Port Jefferson Branch from Northport east to Port Jefferson.

[1][3][7] The trestle's construction was critical in extending the LIRR east to Port Jefferson, as the Nissequogue River and the valley prevented the tracks from running at-grade.

[4] With the opening of the bridge, LIRR extension, and the community's nearby train station, Smithtown – at the time largely consisting of farms – experienced growth and gained a rail connection to New York City to the west and Port Jefferson to the east.