William John Gruffydd (14 February 1881 – 29 September 1954) was a Welsh academic, poet, writer, and politician.
Gruffydd was born in Gorffwysfa, Bethel, in the parish of Llanddeiniolen, Caernarfonshire the son of a quarryman. He went to Bethel primary school and then Caernarfon School followed by Jesus College, Oxford. In 1909 he married Gwenda Evans the daughter of a minister of religion from Abercarn. They had one son. During the First World War he served in the Royal Naval Volunteer Reserve in the North Sea and in Egypt attaining the rank of Lieutenant.
Gruffydd specialised in Celtic culture. He became a schoolteacher and worked in Scarborough and then for two years at Beaumaris Grammar School before taking a post as assistant lecturer in Celtic studies at University College, Cardiff in 1906; from 1918 until 1946 he was Professor of Celtic. He was President of Council of the National Eisteddfod of Wales and edited Y Llenor ('The Littérateur', a highly influential Welsh language journal of literature published by the university). He wrote poetry and prose and contributed to Welsh scholarship by publishing important histories of Welsh literature and legend.
Gruffydd took an interest in Welsh politics and social questions, coming from a nonconformist, radical family. He was a member of Plaid Cymru and served as deputy vice-president in 1937. However, Gruffydd voiced disagreement with Plaid Cymru party president Saunders Lewis, which eventually led to his leaving the party.
Gruffydd was elected to Parliament as a Liberal Member of Parliament (MP) for the University of Wales seat on 29 January 1943 after the sitting MP Ernest Evans became a County Court Judge. Gruffydd's opponent in the by-election was Plaid's Saunders Lewis, and he had effectively split the Welsh-speaking community. He was comfortably re-elected in the 1945 general election and sat until the abolition of University seats in 1950. He did not stand again for Parliament.