Traeth Mawr (Welsh for 'large beach') is a polder near Porthmadog in Gwynedd in Wales. The area was formerly the large tidal estuary of the Afon Glaslyn. It was created after large-scale land reclamation occurred in the late 18th and early 19th centuries. A large embankment, called the Cob, separates the area from the sea and carries a road and railway line.