State Route 99 or SR-99 is a 21.914-mile (35.267 km) long route that extends from Athens in Limestone County to just northeast of Anderson in Lauderdale County.

The southern terminus of SR-99 is located at its intersection with U.S. Highway 31 in northeast Athens. From this point, the route travels in a westerly direction through the city before heading in a northwesterly trajectory towards Mount Rozell. From here it then travels in a westerly direction before terminating at State Route 207 northeast of Anderson.