U.S. Route 51 (US 51), mostly overlapped by the unsigned State Route 3 (SR 3), is a north–south state highway in the U.S. state of Tennessee, that is 135.9 miles (218.71 km) long, completely within West Tennessee. It begins in Shelby County and ends in Obion County. The SR 3 designation is seen largely on mileposts.

Shelby County is the westernmost county in the U.S. state of Tennessee. As of the 2020 census, the population was 929,744. It is the largest of the state's 95 counties, both in terms of population and geographic area. Its county seat is Memphis, a port on the Mississippi River and the second most populous city in Tennessee. The county was named for Governor Isaac Shelby (1750–1826) of Kentucky. It is one of only two remaining counties in Tennessee with a majority African American population, along with Haywood County.