Knucklas (Welsh: Cnwclas, meaning "green hillock") is a village in Powys, Wales, previously Radnorshire.

It lies in the upper valley of the River Teme, just off the B4355 road and is served by Knucklas railway station on the Heart of Wales Line.

Below the castle lies the battlefield of the Battle of Beguildy thought to have been fought between the Welsh and the Mortimer family of Norman Marcher Lords in 1146.

[citation needed] Whilst there is a romantic story associating the castle location with the marriage of Guinevere and King Arthur,[4] this probably developed from an earlier story, which suggested that a marriage took place between Gwenhwyfar, the daughter of Ogrfan Gawr (also called 'Gogrfan Gawr "the Giant" of Castell y Cnwclas' – Knucklas Castle), and Arthur the warrior – there being no reference to Arthur as a king in the early Welsh texts.

The spectacular 13-arch span was completed by the Central Wales Railway in 1865 and recorded in an engraving from the Illustrated London News.