Reginald's Tower, Fortified tower in Waterford, Ireland.

Reginald's Tower is a 54-foot-high cylindrical tower of rough grey and brown rubble masonry, 42½ feet in diameter, with walls 10 feet thick at the base tapering to 7 feet. Its battered base rises to a rounded, slightly overhanging top, and three small rectangular windows with stone surrounds are spaced vertically down the centre-right, with a larger window near the top and narrow slit openings. An arched doorway with a dark recess sits near the bottom left, beside a wall plaque on a carved corbel.

The present tower was probably built between 1253 and 1280, replacing an earlier Viking fortification; in 1185 Prince John ordered the rebuilding of Waterford's defences. It served as a mint, prison and military storehouse, and in 1463 the Irish Parliament ordered coins struck there bearing "Civitas Waterford". In 1495 its cannons sank a ship of Perkin Warbeck during an eleven-day siege, the first successful use of artillery by an Irish city.

The tower displays Viking artifacts from archaeological excavations that visitors can see directly. These finds give a sense of how Vikings lived and traded in this region.

The tower sits at the eastern end of the waterfront and is easy to reach from town. Visitors should know that interior access is through narrow spiral stairs and the building is not fully accessible for those with mobility limitations.

The tower's name comes from Raghnall, a Gaelicised form of the Old Norse Røgnvaldr, likely referring to Ragnall Mac Gilla Muire, the last Hiberno-Norse ruler of Waterford. It is the only urban monument in Ireland to keep a Norse or Viking name.