Hay-on-Wye (Welsh: Y Gelli Gandryll or Y Gelli), often described as "the town of books", is a small market town in Powys, Wales.
The town lies on the River Wye, which is the natural and administrative border with England, and lies within the Brecon Beacons National Park just north of the Black Mountains, Wales. Hay has approximately 1,900 inhabitants. The nearest city is Hereford, some 22 miles (35 km) to the east.

The town was formerly served by Hay-on-Wye railway station, which closed in 1963 under the infamous Beeching Axe.
Hay-on-Wye is a destination for bibliophiles in the United Kingdom, with over thirty bookshops, many selling specialist and second-hand books.

Hay on Wye, like Builth Wells, has two castles within a short distance of each other. It seems likely that Hay was fortified by William Fitz Osbern during his penetration of south-east Wales in the summer of 1070 when he defeated three Welsh kings. The history of the site then continues through the lordships of the Neufmarchés, which was confirmed at the Battle of Brecon in 1093, and also the Gloucester/Hereford families until 1165, when the district of Brycheiniog passed into the hands of the Braoses.

In 1230 Hay Castle passed to the Bohuns and the local history, including the battle near Hay in 1231, is continued through the Mortimer Wars of the 1260s and the battle near Brecon in 1266 down to the death of Earl Humphrey Bohun in 1298.

Lying close to St.Mary’s Church on the western edge of Hay-on-Wye is a small but well-preserved motte. The site overlooks a gorge and small stream leading to the River Wye, which was undoubtedly one reason for the construction of a castle here. A recently levelled platform under the car park to the north east may have once have housed the castle's bailey. This little fortress was probably the work of William Revel, a knight of Bernard de Neufmarché, and may later have been the seat for the manor/commote of Melinog. Other than this the motte has no further recorded history.

The main fortress within Hay-on-Wye was situated on the great site commanding the town and river under the current ruins of the castle and mansion. This was undoubtedly the 'castello de haia' handed to Miles Gloucester in 1121 with the daughter of Bernard Neufmarché. It is most likely that the keep stood by this time. It is therefore possible that this is the oldest Norman tower in Wales, dating to the onslaught of William Fitz Osbern in 1070. During the anarchy (1136-54) in the reign of King Stephen a series of charters were passed by the Gloucesters concerning the castle. In 1165 the last of Miles Gloucester's male descendants was killed at nearby Bronllys Castle and Hay-on-Wye Castle passed into the hands of William de Braose of New Radnor and Buellt. The Braose were energetic lords and probably built the core of the gatehouse which now stands besides the keep. In the summer of 1198 a major English army formed here before marching off to victory at the Battle of Painscastle some four miles to the north.