William Fitzjohn (died 15 September 1326) was a leading prelate in early fourteenth-century Ireland. He held the offices of Bishop of Ossory, Archbishop of Cashel, and Lord Chancellor of Ireland. He is chiefly remembered now for building the town walls of Cashel. His last years were troubled, as he quarrelled with both the English Crown and the Pope. He complained constantly about his poverty. He faced accusations, many of them clearly false, of corruption and immoral living.