The White Book of Rhydderch (Welsh: Llyfr Gwyn Rhydderch, National Library of Wales, Peniarth MS 4-5) is one of the most notable and celebrated surviving manuscripts in Welsh. Mostly written in southwest Wales in the middle of the 14th century (c. 1350) it is the earliest collection of Welsh prose texts, though it also contains some examples of early Welsh poetry. It is now part of the collection of the National Library of Wales, having been preserved in the library at Hengwrt, near Dolgellau, Gwynedd, of the 17th century antiquary Robert Vaughan, who inherited it from the calligrapher John Jones and passed it to his descendants. The collection later passed to the newly established National Library of Wales as the Peniarth or Hengwrt-Peniarth Manuscripts.
What was one manuscript was divided into two in the medieval period and has been bound as two separate volumes, known as Peniarth MS 4 and Peniarth MS 5. Peniarth MS 4 contains the most important material: medieval Welsh tales now collectively known as the Mabinogion. Peniarth MS 5 (the first part of the original manuscript) contains Christian religious texts in Welsh, mostly translated from Latin and French, including Lives of various saints and a tale of Charlemagne.