SR 125 begins as the South Bay Expressway toll road at an interchange with SR 11 and SR 905, although the southbound roadway ends just to the north at Otay Mesa Road, the former alignment of SR 905. The route heads north through an undeveloped area, encountering its first toll plaza just before entering the city of Chula Vista. The freeway cuts through the Eastlake neighborhood of Chula Vista before leaving the city and turning to the northeast, passing through Sweetwater Summit Regional Park near Sweetwater Reservoir. SR 125 intersects with SR 54, where the tolled portion ends. SR 54 runs concurrently with SR 125 north to the Jamacha Road exit. SR 125 continues north again through the unincorporated area of La Presa. The freeway briefly enters Lemon Grove before intersecting with SR 94 and crossing into the city of La Mesa.