Cadair Idris (Welsh pronunciation^ⓘ) or Cader Idris is a mountain in the Meirionnydd area of Gwynedd, Wales. It lies at the southern end of the Snowdonia National Park near the town of Dolgellau. The peak, which is one of the most popular in Wales for walkers and hikers, is composed largely of Ordovician igneous rocks, with classic glacial erosion features such as cwms, moraines, striated rocks, and roches moutonnées.

Cadair Idris means 'Idris's Chair'. Idris is usually taken to be the name of a giant or, alternatively, it may refer to Idris ap Gwyddno (or Gweiddno), a 7th-century prince of Meirionnydd who won a battle against the Irish on the mountain. Idris ap Gwyddno was in fact referred to as Idris Gawr ('Idris the Giant') in some mediaeval genealogies of Meirionydd.

The basic meaning of the word cadair (Middle Welsh/Early Modern Welsh kadeir or cadeir) is 'seat, chair' (borrowed from the Greek cathedra, καθέδρα, 'chair'). In place names cadair can mean 'stronghold, fort, fortress' or 'mountain or hill shaped like a chair'. The spelling cader represents a spoken variant of the standard form cadair.

It appears that Cadair/Cadeir Idris is the form used in the earliest Welsh-language sources. In a poem in his own hand in the second half of the 15th century, the poet Lewys Glyn Cothi wrote "Dros gadair idris gedy" ('and then over Cadair Idris'). Around 1600, John Jones of Gellilyfdy referred to "y mynydh neu bhan neu bhoel a elwir Cadeir Idris" ('the mountain, peak or hill known as Cadeir Idris').

The spoken form represented by cader had developed by the end of the Middle Ages and as a result the form Cader Idris was often used in English and Latin documents at that time and in more recent documents in English and Welsh.

In his Dictionarum Duplex published in 1632, John Davies of Mallwyd differentiated between the two words cadair ('chair') and cader ('fort, fortress'), referring to Cader Idris and Cader Ddinmael. That interpretation was followed by some later lexicographers, including Thomas Charles and Titus Lewis. It is now accepted, however, that cader is a spelling of a spoken form of cadair, and that there is no evidence that cadair and cader are separate words. But as it represents the pronunciation in the local dialect, the spelling Cader Idris is often seen in Welsh and English, and in June 2016 Snowdonia National Park decided to adopt that spelling on its signage, despite advice from the Welsh Language Commissioner and from Park officers, who favoured Cadair. The local dialect form is also seen in the name of the local secondary school, Ysgol y Gader. The name of the mountain is typically spelt 'Cadair Idris on current maps. The summit of the mountain is known as Penygader ('top of the chair/stronghold').

There are three main trails that lead to the top of Cadair Idris. The summit, which is covered in scree, is marked by a trig point. There is also a low-standing stone shelter with a roof.

The Pony Path (Welsh: Llwybr Pilin Pwn), which begins in the north from either Dolgellau or the Mawddach estuary, is the easiest but the longest of the main trails. Its length from the mountain's base is 6 kilometres (3.7 mi) with a 727-metre (2,385 ft) climb.