Mississippi lies entirely within two lowland plains. Extending eastward from the Mississippi River, the Mississippi Alluvial Plain, popularly known as the Delta, is very narrow south of Vicksburg but stretches as much as a third of the way across the state farther north. The Gulf Coastal Plain, covering the rest of the state, includes several subregions, of which the Red Clay Hills of north-central Mississippi and the Piney Woods of the south and southeast are the most extensive. Mississippi's generally hilly landscape ascends from sea level at the Gulf of Mexico to reach its maximum elevation, 806 ft (246 m), at Woodall Mountain, in the extreme northeastern corner of the state.

The state's largest lakes—Grenada, Sardis, Enid, and Arkabutla—are all manmade. Numerous smaller lakes—called oxbow lakes because of their curved shape—extend along the western edge of the state; once part of the Mississippi River, they were formed when the river changed its course. Mississippi's longest inland river, the Pearl, flows about 490 mi (790 km) from the eastern center of the state to the Gulf of Mexico, its lower reaches forming part of the border with Louisiana. The Big Black River, some 330 mi (530 km) long, begins in the northeast and cuts diagonally across the state, joining the Mississippi about 20 mi (32 km) below Vicksburg. Formed by the confluence of the Tallahatchie and Yalobusha rivers at Greenwood, the Yazoo flows 189 mi (304 km) southwest to the Mississippi just above Vicksburg.