Hay-on-Wye (Welsh: Y Gelli Gandryll or Y Gelli), often abbreviated to just "Hay", is a small market town and community in the traditional county and district of Brecknockshire in Wales, currently administered as part of the unitary authority of Powys. Often described as "the town of books", it is the National Book Town of Wales. The annual Hay Festival is a major literary festival.
The settlement's name is first referred to between 1135 and 1147 as "Haya"; in 1299 the name of "La Haye" is used. By the 16th century it was simply called "Hay", and the use of the river as a suffix is a later addition. In 1215, a Welsh name, "Gelli" was recorded, and "Gelli gandrell" in 1614; the two names may have been used concurrently in 1625. The English language name, "Hay", is derived from Old English "hæg", possibly meaning a "fenced area" and a noun used in late Saxon and Norman times for an enclosure in a forest. The Welsh word celli (lenited to Gelli) has a range of meanings including wooded areas of various extents.
The town lies on the south-east bank of the River Wye and is within the north-easternmost tip of the Brecon Beacons National Park, just north of the Black Mountains. The town is situated just within the Welsh side of the border with Herefordshire, England, here defined by the Dulas Brook. Where the brook joins the River Wye just north of the town, the border continues northwards along the river. The Wye was the boundary between the former counties and districts of Radnorshire and Brecknockshire.
The adjacent village of Cusop lies on the English side of the Dulas Brook. The nearest city is Hereford, county town of Herefordshire, some 22 miles (35 km) to the east.
The Royal Mail depot in Hay is a sub-office of Hereford, England and has the HR3 postcode.
Hay-on-Wye is a destination for bibliophiles in the United Kingdom, still with two dozen bookshops, many selling specialist and second-hand books, although the number has declined sharply in recent years, many becoming general antique shops and similar.