State Route 21 (SR 21) is a state highway that runs south-to-north through portions of Chatham, Effingham, Screven, and Jenkins counties in eastern Georgia. The route runs from its southern terminus at SR 204 in Savannah to its northern terminus at US 25 and SR 121 in Millen.

SR 21 begins at SR 204, also known as Abercorn Street, in Savannah. The route travels to the west, becoming a freeway, and gains the designation of I-516 and the unsigned SR 421. The routes travel west, then turn to the north at the interchange with Veterans Parkway. At exit 3, US 17/SR 25 join the freeway from the southwest, and US 80/SR 26 join the freeway from the northeast. Continuing north, the routes have an interchange at I-16, at which point, US 17 departs to the east, concurrent with I-16. At exit 7, US 80/SR 26 depart to the west, and the routes turn northwestward. After an interchange with SR 25, I-516/SR 421 end, and SR 21 continues its northward route. In Port Wentworth, SR 30 joins the route; north of their interchange with I-95, SR 30 departs to the west.

SR 21 continues north, leaving Chatham County and entering Effingham County. The route passes through Rincon and Springfield, then continues northwest, through rural parts of the county, and continuing into Screven County. After a brief concurrency with US 301 around Sylvania, SR 21 continues west into Jenkins County. East of Millen, SR 21 turns to the north at its junction with SR 67, then arcs to the northeast of Millen to meet its northern terminus at US 25/SR 121. SR 21 is being considered for part of the proposed I-3[citation needed].

SR 21 is a major corridor in the areas it passes through. The route handles an Average Annual Daily Traffic of 51,200 vehicles near its southern terminus. While the traffic on the route decreases as the route runs north from Savannah, the route maintains an AADT of at least 2,000 to Sylvania.

The Tom Triplett Parkway is a section of SR 21 located in Port Wentworth on the west side of the Greater Savannah Area.

It stretches from the Effingham-Chatham county line (roughly the Lake Cherie Road intersection) to the Port Wentworth-Garden City municipal border (roughly the SR 307 intersection).

In 2000, the Georgia General Assembly passed a resolution to designate this portion of SR 21 in honor of Tom Triplett, a Democrat who served as Mayor of Port Wentworth and as a State Representative for 18 years. Tom Triplett died in 2006, at 71 years of age.

SR 21 has several banner routes:

State Route 21 Business (SR 21 BUS) is the former alignment of SR 21 through Sylvania.

State Route 21 Connector (SR 21 CONN) is a short connector in Sylvania.

State Route 21 Spur (SR 21 Spur) is a short spur in Garden City connecting SR 21 with SR 25 and several port terminals on the Savannah River.

State Route 21 Spur (SR 21 Spur) is a short spur in Effingham County connecting SR 21 with SR 119 just north of Springfield.