William Mathias (November 1, 1934 — July 29, 1992) was a Welsh composer. Mathias was born in Whitland, Carmarthenshire. A child prodigy, he started playing the piano aged three, and composing aged five. He studied at the Royal Academy of Music under Lennox Berkeley, where he was elected a Fellow in 1965. In 1968, he received the Bax Society Prize of the Harriet Cohen International Music Award. He was professor of music and head of department at the University of Wales, Bangor from 1970 until 1988.