The B377 is a short road on the western edge of London.
It starts at a mini roundabout junction with the B376 in Laleham and goes northeastwards between Staines and the Queen Mary Reservoir.
It soon reaches a mini-roundabout where it TOTSOs right. At this point it meets and follows the pre-bypass A308; the current route of that road is reached a short distance later where the B377 crosses the cuttent A308 at a roundabout. This point is the original southern end of the B377; it was extended south to Laleham in 1935.
The B377 continues north into Ashford and turns right at a set of traffic lights by the town's war memorial. The angel on the memorial has her back to us. The B377 then forms a multiplex along the B378 through the town.
After regaining its number the B377 leaves Ashford via two roundabouts before continuing east into Lower Feltham where it terminates at traffic lights on the A244.