Rhuddlan Castle (Welsh: Castell Rhuddlan; Welsh pronunciation: [kastɛɬ ˈr̥ɨðlan]) is a castle located in Rhuddlan, Denbighshire, Wales. It was chosen as the location of a castle by Edward I in 1277.

Much of the work was overseen by master mason James of Saint George. Rhuddlan, which was not completed until 1282, was built concurrently with Flint Castle, at a time when King Edward I of England was consolidating his conquest of Wales. It was temporarily his residence, and his daughter, Elizabeth, is presumed to have been born there. It is currently owned by Cadw, a division of the Welsh Government.

Rhuddlan was planned as a concentric castle. It has a unique 'diamond' in layout as the gatehouses are positioned at the corners of the square baileys instead of along the sides like at Flint, Harlech or Beaumaris. Records of construction costs show that it was the major piece of building work being carried out by the English during the late 1270s.

The inner ward has defensive walls with twin-tower gatehouses. The outer ward is surrounded by a curtain wall that has small towers and turrets. According to Thomas Pennant, who passed through the town on his travels in the 18th century, one of the towers is named "Twr-y-Silod" ("Grain Tower"), and another "Twr-y-Brenin" ("The King's Tower"). The castle had a three-sided moat, with the River Clwyd protecting its fourth side. Within the inner ward there was a great hall, kitchens, private apartments and a chapel. The outer bailey had a granary, stables and a smithy.

Rhuddlan is next to the River Clwyd. During the fortification's lengthy construction, the river course was straightened and dredged to allow ships to sail inland along a man-made channel. Its purpose was to allow provisions and troops to reach the castle even if hostile forces or a siege prevented overland travel.

A further protective earthwork and timber structure was created around it in 1280–82. The castle was completed prior to the rebellion of Llywelyn the Last in 1282.

The story of Rhuddlan goes back much further than the fortress built by Edward I. Prior to the Norman occupation of lower Gwynedd, Rhuddlan was at the heart of a Welsh cantref. From here the Lords of Rhuddlan commanded the Perfeddwlad (lands of north-east Wales) on behalf of Gruffydd ap Llywelyn (1007 – 5 August 1063), the last ruler of all Wales. The town itself, however, began as a Saxon "burgh" founded by Edward the Elder.

In the late 11th century, the Normans invaded Gwynedd. Rhuddlan's strategic position ensured that it was fought over by the Princes of Gwynedd and the Earls of Chester, with Gruffydd ap Llywelyn, who had been driven out by Harold Godwinson, re-taking the town. The remains of a Norman castle at Twthill, built in 1086, is just to the south of the current castle; it was built by Robert of Rhuddlan, a supporter of King William I of England.