State Route 120 (SR 120) is a state highway that runs west-to-east through portions of Haralson, Paulding, Cobb, Fulton, and Gwinnett counties in northwestern Georgia. The route runs from its western terminus at SR 100 in Tallapoosa to its eastern terminus at US 29 and SR 8 in Lawrenceville.

The route is briefly co-signed with SR 113 east of Buchanan, with US 278 and SR 6 south and east of Dallas, and with SR 9 and SR 140 in northern Fulton County around Alpharetta. It crosses I-75 just east of Marietta, crosses SR 19/Georgia-400 just east of Alpharetta, and I-85 west of its eastern terminus.

From 1969 to 2007, SR 120 Loop looped around Marietta. In October 2007, SR 120 Loop was decommissioned, and mainline SR 120 was rerouted to the southern part of the loop. Prior to October 2007, mainline SR 120 ran through downtown Marietta and formed the southern part of Marietta Square. The portion of SR 120 from U.S. 41/SR 3 to SR 120 Alternate was renumbered SR 3 Connector. This was done at the same time changes were made to SR 5.