SC 128 runs in a west-to-east direction and is a four-lane road for the entirety of the route. SC 128 travels along the entirety of Savannah Highway. Beginning at an intersection with South Carolina Highway 170 just before the Broad River Bridge, SC 128 travels east towards its eastern terminus at U.S. Highway 21.
Until 2012, SC 128 was signed as South Carolina Highway 802. With the re-routing of US 21 around Beaufort, the route for SC 802 was truncated to its current location on Lady's Island, thus resulting in the designation of SC 128 for the remaining portion of the old SC 802 route.