The Penrhyn Quarry Railway was a narrow-gauge railway in Caernarfonshire, Wales. It served the Penrhyn quarry near Bethesda, taking their slate produce to Port Penrhyn, near Bangor. The railway was around six miles (9.7 km) long and used a gauge of 1 ft 10+3⁄4 in.

The Penrhyn quarry is a slate quarry located near Bethesda, North Wales. At the end of the nineteenth century it was the world's largest slate quarry; the main pit is nearly 1 mile (1.6 km) long and 1,200 feet deep, and it was worked by nearly 3,000 quarrymen. It has since been superseded in size by slate quarries in China, Spain and the USA. Penrhyn is still Britain's largest slate quarry but its workforce is now nearer 200.