Sir William St Leger (1586 – 2 July 1642) was an Anglo-Irish landowner, official and soldier active in Ireland.
He was the son of Sir Warham St Leger (died 1600) and his wife Elizabeth Rothe of Kilkenny, and a grandson of Anthony St Leger, Lord Deputy of Ireland. He was probably born in County Cork. When he was 14 his father and Hugh Maguire (Lord of Fermanagh) fatally wounded each other in single combat near Cork city.
He took part in the Flight of the Earls in 1607, when Hugh O'Neill, Earl of Tyrone, and Rory O'Donnell, 1st Earl of Tyrconnell, together with more than ninety of their family and followers, the chief of the Gaelic and Catholic resistance in Ireland, fled to Europe. He later said that his involvement in the Flight was accidental, in that he had sought the protection of Tyrone from legal proceedings being taken against him, and fled with him because he had nowhere else to go. Sir William spent several years abroad, living firstly in Brussels and then Dordrecht, where he married his first wife Gertrude de Vries.
Having received a pardon from King James I and extensive grants of land in Ireland, he was appointed President of Munster by Charles I in 1627. He warmly supported the arbitrary government of Thomas Wentworth, Earl of Strafford, actively assisting in raising and drilling the Irish levies destined for the service of the king against the Parliament. He was a member of the Irish House of Commons from 1634, as MP for County Cork.