Pentre-llyn-Cymer nestles between two large waters in the county of Conwy, North Wales, the first being Alwen Reservoir and the latter being Lake Brenig. Indeed, Pentre-llyn-Cymer sits on the 'junction' where the River Brenig meets the River Alwen. The Alwen then travels on to join the River Dee just outside the village of Cenwyd in Denbighshire.
The Lake Brenig Visitor Centre is 4km north east of the village along the side of the lake, where canoeing and walking routes are available, whilst the Clwydian Way footpath also passes straight through Pentre-llyn-Cymer and onwards towards the lake. The Pentre-llyn-Cymer Outdoor Education Centre is popular with school groups during the summer months.
To the north of the hamlet are the bleak Denbigh Moors and Mynydd Hiraethog, with the Glocaenog Forest to the east, the (519m) mountain of Marial Gwyn 5km north east of the village, and the hills of Mynydd Poeth (417m) and Fridd (388m) to the south west.
Lake Brenig is renowned for: lakeside walks; mountain bike trails, both around the lake and through the forests; and for the wealth of historic sites to be found in the vicinity.
There are remains of ancient settlements less than 1km to the west of the village and several cairns and tumuli in the surrounding area. Indeed the Brenig Visitor Centre promotes a popular archaeological trail around the perimeter of the lake.