The town is twinned with Redu, a village in the Belgian municipality of Libin,[3] and with Timbuktu, Mali, West Africa.

[9] 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 just on the Welsh side of the border with Herefordshire, England, here defined by the Dulas Brook.

The nearest city is Hereford, county town of Herefordshire, some 22 miles (35 km) to the east.

[12] Hay-on-Wye is in the area known as "Kilvert country" which includes Clyro, Capel-y-ffin, Llowes, Glasbury, Llanigon, Painscastle, Clifford and Whitney-on-Wye.

The Welsh word celli (lenited to Gelli) has a range of meanings, including wooded areas of various extents.

[18] In 1947 the General Post Office changed the name of the postal locality from Hay to Hay-on-Wye.

[28][29] The main part of the town was subsequently developed on a spur of land between the River Wye and Dulas Brook, about 200 metres east of Hay Tump.

The town was fortified with walls and defended by Hay Castle,[30] which appears to have been built from the late 12th century onwards.

The earlier centre of settlement around Hay Tump and St Mary's Church lay outside the later town walls.

The Welshry was outside the fortified town, including some rural land, the village of Llanigon and the hamlet of Glynfach.

[44][45] Hay-on-Wye is a destination for bibliophiles in the United Kingdom, with two dozen bookshops, many selling specialist and second-hand books,[46] although the number has declined sharply in recent years, many becoming general antique shops and similar.

Richard Booth opened his first shop there, called The Old Fire Station, in 1962,[48][49] and by the 1970s Hay had gained the nickname "The Town of Books".

[50] Since 1988, the Hay Festival based in Hay-on-Wye has been the venue for an annual literary festival, now sponsored by The Daily Telegraph newspaper, which draws a claimed 80,000 visitors over ten days at the end of May or beginning of June to see and hear big literary names from all over the world.

Devised by Norman, Rhoda and Peter Florence in 1988, the festival was described by Bill Clinton in 2001 as "The Woodstock of the mind".

[65][66] The town was formerly served by train services at Hay-on-Wye railway station[67] on the Hereford, Hay and Brecon line.

The driver George Parker died, and his stoker John Williams had life changing injuries.

[95] Oakfield is a Grade II listed Regency house located south of the town centre: built in about 1820, it was recorded in 1842 as the home of Henry Allen Junior.

The bell was paid by a donation as a memorial to T. W. Higgins, Hay, and Guidfa House, Radnorshire.

[65] On 1 April 1977, bibliophile Richard Booth conceived a publicity stunt in which he declared Hay-on-Wye to be an 'independent kingdom' with himself as its monarch and a National Anthem written by Les Penning.

The tongue-in-cheek micronation of Hay-on-Wye has subsequently developed a healthy tourism industry based on literary interests for which some credit Booth.

[131] In 2005, Booth announced plans to sell his bookshop and move to Germany; on this occasion, local Member of Parliament, Roger Williams, was quoted as saying: "His legacy will be that Hay changed from a small market town into a mecca for second-hand book lovers and this transformed the local economy.