Prydain or Prydein is the Welsh name of the island of Britain.

In mediaeval texts (e.g. the Mabinogion), the term often refers to the northenmost part of the island, beyond the Forth and Clyde: where the island as a whole is meant, the phrase Ynys Prydain (Isle of Prydain) is commonly used. Compare Albion and Alba for a similar distinction.

The name has been used by a number of authors for ancient Celtic or fantasy lands, including numerous works of Stephen Lawhead, and Lloyd Alexander's Chronicles of Prydain.