The Penrhyn Slate Quarry is a slate quarry located near Bethesda in 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 (370 metres) 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 It has since been superseded in size by slate quarries in China, Spain and the USA.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.
The quarry was first developed in the 1770s by Richard Pennant, later Baron Penrhyn, although it is likely that small-scale slate extraction on the site began considerably earlier. Much of this early working was for domestic use only as no large scale transport infrastructure was developed until Pennant's involvement. From then on, slates from the quarry were transported to the sea at Port Penrhyn on the narrow gauge Penrhyn Quarry Railway built in 1798, one of the earliest railway lines. In the 19th century the Penrhyn Quarry, along with the Dinorwic Quarry, dominated the Welsh slate industry.
The quarry holds a significant place in the history of the British Labour Movement as the site of two prolonged strikes by workers demanding better pay and safer conditions. The first strike lasted eleven months in 1896. The second began on 22 November 1900 and lasted for three years. Known as "The Great Strike of Penrhyn" this was the longest dispute in British industrial history. In the longer term the dispute cast the shadow of unreliability on the North Welsh slate industry, causing orders to drop sharply and thousands of workers to be laid off.
From 1964 until 2007 it was owned and operated by Alfred McAlpine PLC.
In 2007 it was purchased by Kevin Lagan (an Irish businessman who is the owner and chairman of the Lagan Group) and renamed Welsh Slate Ltd. Kevin Lagan and his son Peter (MD of Lagan Building Solutions Ltd) are now directors of Welsh Slate Ltd which also includes the Oakeley quarry in Blaenau Ffestiniog, the Cwt Y Bugail quarry and the Pen Yr Orsedd quarry.