William Thomas Cosgrave (6 June 1880 – 16 November 1965) was an Irish Fine Gael politician who served as President of the Executive Council from 1922 to 1932, Leader of the Opposition from 1932 to 1944, Leader of Fine Gael from 1934 to 1944, Leader of Cumann na nGaedheal from 1923 to 1933, Chairman of the Provisional Government from August 1922 to December 1922, President of Dáil Éireann from September 1922 to December 1922, Minister for Finance from 1922 to 1923 and Minister for Local Government from 1919 to 1922.
He served as a Teachta Dála (TD) from 1921 to 1944.
He was a Member of Parliament (MP) for the North Kilkenny constituency from 1918 to 1922.Cosgrave never officially held the office of Taoiseach (the current title of Ireland's head of government, created in 1937); however, as the first elected head of government in the Irish Free State, he is recognised to have been the first Taoiseach.
His son Liam Cosgrave went on to serve as Taoiseach from 1973 to 1977.