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. He was born in Carneddi, Bethesda, Caernarfonshire, Wales, and studied Latin and Welsh at the University College of North Wales, Bangor. He was appointed Head of the Department of Celtic at the University of Liverpool in 1936. His academic career was interrupted by World War II, when he worked in Naval Intelligence, contributing to the Geographical Handbook Series. He was appointed as the third holder of the Oxford chair in Celtic (originally held by Sir John Rhys) in 1947 and became a Fellow of Jesus College, Oxford at the same time. He was knighted in 1977 and retired the following year back to his birthplace, where he died in 1984.