The B4603 follows the original route of the A4067 along the right bank of the Tawe in the lower part of the Swansea Valley.
Leaving the present line of the A4067 at the Cilmaengwyn roundabout, Ynysmeudwy, it runs downstream through the valley communities of Pontardawe, Trebanos, Clydach and Ynystawe to reach Ynysforgan, where it meets the M4 at J45. It then runs into Swansea through Morriston, Plasmarl and Landore to end at a junction with the A483 at the southern end of Neath Road, Hafod, a kilometre north of Swansea city centre.
