Cefn is a community in the County Borough of Wrexham in Wales. It is part of the ecclesiastical parish of Rhosymedre. The community is made up of the villages of Cefn Mawr, Cefn Bychan, Newbridge, Acrefair, Penybryn and Plas Madog.
Cefn Mawr was part of the ancient parish of Ruabon and the area was known as Cristionydd Cynrig (or Cristionydd Kenrick in English). In 1844, most of Cristionydd Cynrig, together with the neighbouring township of Coed Cristionydd became part of the new parish of Rhosymedre.
Cefn was formerly a heavily industrialised area, with large deposits of iron, coal and sandstone, and heavy industry dominated the landscape in the 18th and 19th centuries. Iron was worked at several blast furnaces and forges throughout the area and coal was dug at pits in Cefn, Plas Kynaston and Dolydd. Stone was cut at quarries above Cefn Mawr. Much of the mineral wealth of the area was exported by canal over the Pontcysyllte Aqueduct on the Shropshire Union Canal until the railway reached Ruabon in 1855.
In 1867 Robert Ferdinand Graesser, an industrial chemist from Saxony, Germany established a chemical works at Plas Kynaston to extract paraffin oil and wax from the local shale. The company later expanded into the production of coal tar, carbolic acid and phenol. The site soon became the world's leading phenol producer. In 1919 the US chemical company Monsanto entered into a partnership with Graesser's chemical works to produce vanillin, salicylic acid, aspirin and later rubber.