The Book of 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. It is made of parchment and was written in Wales around 1265, probably in a monastery, would previously have been kept alive through oral tradition. The best-known poem contained within its pages is Y Gododdin, an early Welsh-language poem commemorating the warriors from Gododdin (Lothian in modern Scotland) who fell at the Battle of Catraeth (probably Catterick in North Yorkshire) around the year 600. Parts of this do appear to be contemporary with Aneirin, who, it is claimed, was a survivor of the battle. Military activities are described in great detail in the poem.
The other poetry, with no connection to this battle, includes, amongst others, "Peis Dinogat", a short poem for a child named Dinogad, in the form of a lullaby, describing how his father goes hunting and fishing. The literary scholar, Sir Ifor Williams, suggested that its incongruous presence within the Book of Aneirin might have been the result of it having been written in the margin of the original manuscript.