State Route 20 (SR 20) is a state route roughly in the shape of a capital J rotated ninety degrees to the left, which runs through portions of Floyd, Bartow, Cherokee, Forsyth, Gwinnett, Walton, Rockdale, Newton, and Henry counties in northwestern and north-central Georgia. Its counterclockwise, or western terminus is at the Alabama state line in Floyd County, and its clockwise, or eastern terminus occurs at its intersection with U.S. Route 19/U.S. Route 41 (US 19/US 41) and SR 3 in Hampton in Henry County at the Atlanta Motor Speedway.

From the Alabama state line, SR 20 proceeds east through central Floyd County into the city of Rome, and is concurrent with US 27, SR 1, and SR 53 through downtown Rome. The highway leaves Rome to the east concurrent with US 411, bisecting Floyd County, and crosses into and bisects Bartow County, still co-signed with US 411until just north of Cartersville (US 41 is briefly concurrent near Cartersville as well), after which SR 20 continues eastward on its own. The route intersects I-75 northeast of Cartersville, and continues to head east, passing Lake Allatoona to its north, and crossing into Cherokee County. SR 20 heads east into central Cherokee County and through its county seat in Canton, and crosses I-575, before continuing east into Forsyth County. In central Forsyth County, the route dips southeast to pass to the south of Sawnee Mountain and heads through Cumming, crossing US 19 and SR 400, and heading on into Gwinnett County, passing to the south of Lake Lanier. SR 20 then turns south after intersecting I-985, crossing I-85 shortly thereafter. The road also has provides access to SR 316, leading to Athens.

SR 20 continues southward through Lawrenceville and heads into Walton County and through Loganville, paralleling the Gwinnett-Walton County line, before crossing into Rockdale County and meeting I-20 just south of Conyers. Likely to help alleviate driver confusion due to two identically-numbered highways meeting, the highway is usually referred to at this point as SR 138, with which it runs concurrent through Conyers. SR 20 continues south and then southwest to the Henry County seat of McDonough, where it briefly runs concurrent with SR 81. SR 81 departs to the west after the routes cross I-75 at exit 218. The portion of SR 20 between the western end of the SR 81 concurrency and US 19/US 41, the route's clockwise/eastern terminus, has recently been widened from two lanes to four, including a new southern bypass of the city of Hampton, and more controlled access at US 19/US 41. The primary purpose of the widening is to facilitate the flow of traffic to and from the Atlanta Motor Speedway located near the highway's eastern terminus.

The Georgia Department of Transportation average annual daily traffic (AADT) numbers for the year 2011 show a variety of daily averages across SR 20. The traffic load on the route starts at its lowest daily average load as the route starts into Floyd County, where numbers hover around 5,400 vehicles per day. These averages increase rapidly as the route approaches Rome, going from around 12,000 vehicles to a peak of 38,000 vehicles in downtown Rome. As SR 20 becomes concurrent with US 27, the numbers decrease to an average of 32,000, and then decrease more rapidly to around 15,000 as SR 20 heads east out of Rome. The vehicle load stays in that area all the way through Floyd County and into Bartow County, as the route is the main west-to-east thoroughfare between Rome and Cartersville. In Cartersville, where the route in concurrent with US 41, the vehicle count reaches a zenith of just over 41,000, but then drops off sharply as the route heads into rural Cherokee County, dipping just below 10,000 before increasing again to near 24,000 vehicles per day as the route approaches I-575. On the portion of the route that is concurrent with I-575, the route sees its maximum vehicle load of just over 54,000 vehicles, again dropping rapidly east of I-575, going from around 24,000 down to 11,000, and further down to around 10,000 as the Forsyth County line is reached.

Numbers start to creep up again as the Forysth county seat of Cumming approaches, going from 10,000 to over 22,000 vehicles, cresting at 37,000 vehicles south of Cumming, as the route feeds traffic onto US 19/SR 400 southbound into Atlanta. Averages stabilize around 20,000 vehicles per day as SR 20 heads into Gwinnett County, briefly cresting again at over 41,000 vehicles around I-985, and dipping to around 30,000 vehicles around I-85. South of Lawrenceville, numbers then start to decrease once more from around 22,000 down to 14,000 around Loganville, and dip down into average of just over 8,000 in rural Walton County. Until the route reaches Conyers, average numbers see lows of 6,800 in Rockdale County, increasing again to just over 31,000 in and around Conyers and I-20. Rapidly decreasing once more to around 7,600 vehicles per day in Newton and northern Henry counties, another high is reached in the vicinity of I-75 with just over 28,000 vehicles per day, before the route reaches its eastern terminus in Hampton with average traffic loads just over 12,000 vehicles per day.

The first portions of the roadway that are signed as SR 20 today makes its appearance on Georgia state road maps in 1920, which show the portion of the route from the Alabama state line through Rome to Carterville signed as SR 4. The section from Cartersville to Cumming running east across north Georgia is missing at the time. The next extant portion was the section running southeast from Cumming to Buford, which appears to be unsigned in 1920, and the piece between Buford and Lawrenceville, which is signed as SR 13. The entire remainder of the current route was not yet in existence in 1920. In 1921, the section between Cumming and Buford is shown as being signed as SR 68, and by 1929, the existing portion from the Alabama state line to Cartersville had been signed as SR 20, co-signed with US 41 W.

By early in 1932, Georgia state road maps show a new graded, but unimproved 18.5 miles (29.8 km) section signed as SR 20 between Rydal in Bartow County, and Canton in Cherokee County, which corresponds to today's routing of SR 140 in the area, and which met SR 20 at its intersection with today's SR 108. In addition, a shorter 10.2 miles (16.4 km) portion of the route between Lawrenceville and Loganville also made its first appearenace. The existing portion of the route between just west of Rome to Cartersville appears as having been finished with hard surface, and the SR 13 section between Buford and Lawrenceville appeared in the same finished condition.

Just a month later, the 23.8 miles (38.3 km) connection between Canton and Cumming is shown as graded but unimproved, and is signed as SR 20. The sections from Cumming through Buford and Lawrenceville to Loganville had also been re-signed as SR 20, and by the middle of the year, the route had been extended yet again with the addition of the portion from Loganville to Conyers. Motorists were now able to travel from the state line with Alabama to Loganville, using SR 61, which was co-signed as SR 20 by October in 1934, to connect to Rydal in lieu of the still-missing portion between Cartersville and Canton.

Nearly two years later saw the next extension, taking the route to from Conyers to McDonough. Outside of some small surface improvements east of Canton, the next significant change in the route was the appearance in early 1941 of the straight west-to-east 23.4 miles (37.7 km) connection between north of Cartersville and Canton, which was initially signed as Route 20 Spur (between US 41 and SR 61) and SR 113, and re-signed to be part of SR 20 by 1943.

By 1953, no additions to the signed route length had been made, but the roadway connecting McDonough with Hampton had been constructed. In addition, surface conditions had been improved to feature hard surface for the entire length of the route, with the exception of portions between Cartersville and Canton, south of Loganville, and north of McDonough. Also by 1953, the SR 20 Spur running south from the main route to Allatoona Dam had been constructed and was signed as SR 294. It was 1960 before this final piece of the route, between McDonough and Hampton, had been finished and designated as part of SR 20. The entirety of the route was now extant, and was covered by hard surface

State Route 20 Spur (SR 20 Spur) runs from SR 20, just east of its intersection with I-75 east of Cartersville, south to Allatoona Dam. The route, along with nearby Allatoona Dam road, formed the two segments of State Route 294.