Gweith Gwen Ystrat (in English: The Battle of Gwen Ystrad, is a late Old Welsh or Middle Welsh heroic poem found uniquely in the Book of Taliesin, where it forms part of the Canu Taliesin, a series of poems attributed to the 6th-century court poet of Rheged, Taliesin. Put in the mouth of a first-person eyewitness, the poem glorifies a victory by Urien, prince of Rheged, in which he led his warband in defence against a host of invaders at a site called Llech Gwen in Gwen Ystrad (Gwen valley). The heavy, prolonged fighting is said to have taken place since dawn at the entrance to a ford. Sir Ifor Williams suggests that the personal name Gwên may lie behind the forms Llech Gwen and possibly Gwen Ystrad, but the site cannot be identified.