The Ceiriog Valley (Welsh: Dyffryn Ceiriog) is the valley of the River Ceiriog in north-east Wales. It is also the name of a ward of the County Borough of Wrexham. The ward is the largest ward of the county borough by area and forms a strikingly-shaped salient of the county borough between Powys and Denbighshire. The valley forms part of the traditional county of Denbighshire; and between 1974 and 1996 it became part of the short-lived county of Clwyd. Part of the lower end of the valley extends into Shropshire, England. The Ceiriog Valley is some 20 km long and runs generally west to east, south of the Vale of Llangollen. The valley receives relatively few tourists despite being only a few miles from the A5 road and being described by British prime minister David Lloyd George as "a little bit of heaven on earth". It is something of a dead end, with the B4500 road terminating at Llanarmon Dyffryn Ceiriog, a village near the head of the valley.
The Ceiriog Valley is divided into three communities: from west to east, Ceiriog Ucha ("Upper Ceiriog"), Llansantffraid Glyn Ceiriog, and Glyntraian. The largest village in the Ceiriog Valley is Glyn Ceiriog (also known as Llansantffraid Glyn Ceiriog). Villages and hamlets in the Ceiriog Valley include:
Llansantffraid Glyn Ceiriog
- Glyn Ceiriog
Three notable Welsh poets have connections with the Ceiriog Valley: John Hughes (1832 – 1887) was born on a farm near Llanarmon Dyffryn Ceiriog; Huw Morus (1622 – 1709) was born and lived near Pandy in the Ceiriog Valley; and Rev. Robert Elis (1812 – 1875) was a Baptist minister in Glyn Ceiriog from 1838 until 1840. (Hughes took the middle name Ceiriog and also used it as his bardic name; Morus's bardic name was Eos Ceiriog – the Nightingale of Ceiriog; and Elis was better known by his bardic name, Cynddelw.) The Ceiriog Memorial Institute in the village of Glyn Ceiriog was built as a memorial to them all, and contains stained glass windows dedicated to each of their memories.
- Dewi Parry Jones & Robert Owen Jones, "100 Years in the Valley - Y Glyn a Fu" (1998)
- Dewi Parry Jones & Robert Owen Jones, "100 Years in the Valley Volume II - Y Glyn a Fu" (1999)