Known locally as Sandy Beach, Porth Tywyn Mawr is a sandy bay 800 yards wide, near the village of Llanfwrog. It lies between the headlands of Trwyn Llwyd (to the north) and Trwyn Cliperau. The shore is of sand, with the rocky islet of Craig Dafydd in the centre of the bay. The Penrhyn Bay camping site occupies much of the southern headland. It's a popular location for most watersports, particularly jet-skiing, windsurfing, canoeing, sailing and surfing.
Some roadside parking is available at the end of the lane from Llanfwrog at Grid Ref : SH289850 (Lat : 53.3342°N, Lon : 4.57081°N). At the bottom of this lane is the coastal path, and access to the beach is a short track. The caravan park has a small shop which is open to the public and sells ice-creams, newspapers, groceries etc. Dogs are allowed at all times. Signs warn of unexpectedly large waves generated by ferry movements at Holyhead.
The name Porth Tywyn Mawr translates as 'Great Warren Bay'.