Mississippi Highway 309 (MS 309) is a two-lane, north–south highway in Marshall County. It runs from the Mississippi–Tennessee state line south through western Marshall County to its southern terminus at MS 4, just east of the community of Chulahoma.
MS 309's northern terminus lies at the Tennessee state line, continuous with Byhalia Road in Shelby County, Tennessee, to the north. MS 309 travels south through the junction with MS 302 at Barton.
Below Barton, MS 309 continues south, crossing the Coldwater River before entering the town of Byhalia; where the highway passes through a grade-separated interchange with U.S. Route 78 (US 78). Thence, MS 309 continues to its southern terminus at MS 4, just east of the community of Chulahoma.