Seán Francis Lemass (15 July 1899 – 11 May 1971) was an Irish Fianna Fáil politician and third Taoiseach of Ireland from 1959 until 1966.

A veteran of the 1916 Easter Rising, the War of Independence and the Civil War, Lemass was first elected as a Sinn Féin Teachta Dála (TD) for the Dublin South constituency in a by-election on 18 November 1924 and was re-elected at each election until his retirement in 1969. He was a founder-member of Fianna Fáil in 1926, and served as Minister for Industry and Commerce, Minister for Supplies, and Tánaiste in successive Fianna Fáil governments.

Lemass is remembered for his tireless work to develop Irish industry and for forging new links between the Republic and Northern Ireland in the 1960s. He is regarded by many in Fianna Fáil (and indeed by Fine Gael Taoisigh Garret FitzGerald and John Bruton) as the finest Taoiseach in the history of the Irish state and as "the architect of modern Ireland."