SC 125 is predominantly a two-lane rural highway, traveling in a northwesterly direction for 56.2 miles (90.4 km).

Beginning in downtown Allendale, it is also known as the Augusta Highway and travels northeast to Martin before entering Barnwell County.

After a 17.3-mile (27.8 km) drive, and well into Aiken County, travelers exit the Savannah River Site and enter the town of Jackson.

Continuing north, on what is now called Atomic Road, SC 125 joins a concurrency with US 278 from Beech Island to Clearwater.

Entering North Augusta, it crosses Interstate 520 (I-520) without an interchange and connects with US 25 before the road ends at Buena Vista Avenue.

[5] Though the highway is maintained by SCDOT, it is patrolled by federal security; the speed limit throughout is 55 miles per hour (89 km/h).

Drivers were required to return the pass at the exit checkpoint; for those that drove too quickly to the other end would be issued a speeding ticket.

There is no access from Jefferson Davis Highway north to East Buena Vista Avenue or vice versa.

It then meets its northern terminus, an intersection with the SC 125 mainline (known as East Buena Vista Avenue west of here and Atomic Road north of here).

At an intersection with the northern terminus of SC 125 (Buena Vista Avenue), the truck route ends, and US 25 Bus.