Cadair Idris或 Cader Idris是北威爾士斯諾多尼亞國家公園的一座山，坐落在國家公元的最南邊，高達893米，被稱為Penygadair（威爾士的“椅頂”）。

Cadair Idris or Cader Idris is a mountain in the Snowdonia National Park, north Wales. It lies at the southern end of the National Park and reaches 893 m at its summit, named Penygadair (Welsh for "top of the chair"). It is some 65km south of Snowdon itself.

To the north lie the town of Dolgellau and the River Mawddach estuary, while to the south is the glaciated Tal-y-llyn Lake. Two miles further west is the eastern end of the Tal-y-llyn Railway, a narrow gauge preserved railway. A number of named paths lead to the summit, such as the Pony Path, or the Fox's Path, the latter leading directly up the northern face of the mountain — a three-mile-long cliff and scree face. In recent years, the Fox's Path has degraded sufficiently to make it a dangerous descent for any other than experienced hikers and scramblers.

The name Cadair Idris means "the chair of Idris" in Welsh, a reference to a giant in Welsh mythology and the resemblance of one of the mountain's cwms, Cwm Cau, to an enormous armchair. The spelling Cader Idris is often found in both Welsh and English, as reflected in the name of the local secondary school, Ysgol y Gader . However, Cadair Idris is the form used on modern maps and many people regard Cader as incorrect.

Cadair Idris is imbued with numerous legends; some nearby lakes are supposed to be bottomless, and anyone who sleeps on its slopes will supposedly awaken either a madman or a poet, a legend associated with several other places in Wales. In Welsh mythology, Cadair Idris is also said to be the hunting ground of Gwyn ap Nudd and his C?n Annwn (hounds of the underworld). The howling of these huge dogs foretold death to anyone who heard them, the pack sweeping up that person's soul and herding it into the underworld.

The crater-like shape of Cwm Cau has given rise to the occasional mistaken claim that Cadair Idris is a recently extinct volcano. This was debunked as long ago as 1872.