State Route 821 (SR 821) is a north–south state highway in the southeastern portion of the U.S. state of Ohio. A state-maintained section of the old US 21, its southern terminus is at SR 60 approximately 2 miles (3.2 km) north of Marietta, and its northern terminus is at I-77 in Byesville, along with the eastern terminus of SR 209. The route is entirely undivided surface road, and is much less direct than the newer I-77 which supplanted it as a through route.