Hay Castle (Welsh: Castell y Gelli) is a medieval fortification and 17th-century mansion house in the small town of Hay-on-Wye in Powys, Wales.

Originally constructed as part of the Norman invasion of Wales, the castle was designed as a ringwork overlooking the town in either the late 11th or the early 12th centuries.

The Normans began to make incursions into South Wales from the late 1060s onwards, pushing westwards from their bases in recently occupied England.

[5] In the late 11th or early 12th century, a new fortification was built inside Hay itself, on high ground around 200 metres (660 ft) from the old motte, taking the form of an earth ringwork with a stone gate tower.

[6] The de Braose dynasty expanded Hay Castle in stone around 1200 with a curtain wall reinforced by intramural timbers, turning the gate tower into a keep.

[10] During the Second Barons' War, Prince Edward captured the castle in 1263 but it was recaptured and burnt by Simon de Montfort and Llywelyn ap Gruffudd the following year.

[11] By the time the antiquarian John Leland visited in the 16th century, the town of Hay was "wonderfully decaied" although the castle was described as having once "bene right stately".

One possibility is that during the first half of the 17th century Howell Gwynne built a manor house to the west of the old keep, which was replaced by a new mansion in 1660 by James Boyle of Hereford.

[13] Another reverses this sequence, suggesting that James Boyle left the castle to Howell Gwynne in 1603, and that the mansion was built at the beginning of the century.

[14] In either case, the Jacobean building was two storeys tall, three with its facade included, and featured seven dormer gables in a Dutch style and a large staircase.