Llyn Traffwll is a 91 acre natural lake located, in Anglesey, Wales. Located midway between the villages of Rhosneigr and Valley it lies less than a kilometre south of the A55 North Wales Expressway. It is the largest natural lake on the island with Llyn Alaw and Llyn Cefni both being reservoirs. Along with the surrounding smaller lakes it is a Site of Special Scientific Interest and is adjacent to the Valley Wetlands RSPB site. Formerly (starting in 1866) the lake was used to supply water to Holyhead by the Holyhead Water Company, and to aid water management a small dam was built which still exists.