The Book of Aneirin (Welsh: Llyfr Aneirin) is a late 13th century Welsh manuscript containing Old and Middle Welsh poetry attributed to the late 6th century Northern Brythonic poet, Aneirin, who is believed to have lived in present-day Scotland. The manuscript is kept at the National Library of Wales, Aberystwyth. It is made of parchment and was written in Wales around 1265, probably in a monastery, but is probably a copy of a lost 9th century original. There is minimal decoration, consisting only of a few coloured capital letters.