Cymerau quarry was a slate quarry served by the Ratgoed Tramway, a horse-worked section of the Corris Railway. It is located about half a mile north of Aberllefenni in Merioneth (now Gwynedd), North Wales, on the eastern side of the isolated Cwm Ceiswyn. It worked the Narrow Vein, the highest-quality slate vein in the Abercorris Group.

Cymerau was started around 1860, following the success of the earlier Ratgoed quarry to the north. Slate was extracted from five underground chambers which were below the water table and were pumped using a waterwheel.

In 1864 the Ratgoed Tramway opened, a 2 ft 3 in (686 mm) gauge horse-worked tramway which connected the Ratgoed and Cymerau quarries with the newly opened Corris Railway at Aberllefenni. This prompted significant expansion work at the quarry. In 1878, the quarry was owned by H. R. Hughes and his nephew.

A second quarry was opened around 1880 between the original Cymerau workings and Ratgoed. This was a series of surface pits, connected to the Ratgoed Tramway by an incline. Slate was taken down to the mill at the southern site for processing.

Cymerau had several private wagons which it used to haul slate along the Ratgoed Tramway and on down the Corris Railway to Machynlleth.

At its peak the quarry employed over 100 men.

Cymerau and Ratgoed were worked as a single operation immediately before the First World War. Both quarries closed down during the First World War.

Cymerau re-opened on a smaller scale after the conflict, owned by the Inigo Jones company of Groeslon. In 1921 it produced just three tons of finished slab, and employed only three men. Fortunes improved modestly during the 1920s, reaching a peak employment of 14 men in 1927.