The Peniarth Manuscripts, also known as the Hengwrt-Peniarth Manuscripts, are a collection of medieval Welsh manuscripts originally collected by Robert Vaughan (c.1592–1667) of Hengwrt, Merionethshire, and in the 19th century housed in Peniarth Mansion, Llanegryn, Merioneth. The collection contains some of the oldest and most important Welsh manuscripts in existence.
The collection was sold by William Wynne VII of Peniarth to Sir John Williams in 1898. When the plan to establish a National Library of Wales emerged, Williams promised that he would donate his valuable collection, including the Peniarth Manuscripts, on condition that it be based in Aberystwyth.
The collection includes treasures such as the Black Book of Carmarthen, Book of Taliesin and White Book of Rhydderch (containing the Four Branches of the Mabinogi, the Three Welsh Romances and other tales) and a number of other ancient manuscripts, including early texts of the Cyfraith Hywel, and the Feirdd yr Uchelwyr. The manuscripts in other languages include two Latin manuscripts of Geoffrey of Monmouth's Historia Regum Britanniae and an early illustrated version of the Canterbury Tales, under the name "Hengwrt Chaucer".
The collection is housed at the National Library of Wales, Aberystwyth.