Sir Idris Llewelyn Foster (23 July 1911 – 18 June 1984) was a Welsh scholar and Jesus Professor of Celtic at the University of Oxford from 1947 until 1978.
His academic career was interrupted by World War II, when he worked in Naval Intelligence, contributing to the Geographical Handbook Series.
His work in the field of Celtic Studies was in Medieval Welsh and Irish, both poetry and prose.
He made a particular contribution to the study of tale of Culhwch ac Olwen, and his edition of the tale was published in 1992, after his death, by the assistance of his friends Rachel Bromwich and D. Simon Evans.
[1] He was involved in many Welsh organisations and societies, particularly the National Eisteddfod, where he was Chairman of the Council and President of the Court, the National Library of Wales (Treasurer and Vice-President), and the Church in Wales (member of the Governing Body).