Seán Lester (28 September 1888, Carrickfergus, County Antrim, Ireland – 13 June 1959, Galway, Ireland) was an Irish diplomat and the last Secretary General of the League of Nations, from 31 August 1940 to 18 April 1946.

He was born in County Antrim, the son of a Protestant grocer. Despite the fact that the town of Carrickfergus, where he was born and raised, was strongly Unionist, he joined the Gaelic League as a youth, and was won over to the cause of Irish nationalism. As a young man he joined the Irish Republican Brotherhood. He was working as a journalist for the North Down Herald and a number of other northern papers, before moving to Dublin, where he found a job with the Freeman's Journal. There, by 1919, he had risen to news editor.

After the War of Independence, a number of his friends joined the new government of the Irish Free State. Lester was offered, and accepted, a position as Director of Publicity. He married Elizabeth Ruth Tyrrell in 1920, by whom he had three daughters.

In 1923 he joined Ireland's Department of External Affairs. He was sent to Geneva in 1929 to replace Michael MacWhite as Ireland's Permanent Delegate to the League of Nations. In 1930 he succeeded in organising Ireland's election to the Council (or executive body) of the League of Nations for a three-year term. Lester often represented Ireland at Council meetings, standing in for the Minister for External Affairs. During this time he became increasingly involved in the work of the League, particularly in its attempts to bring a resolution to two wars in South America. This work brought him to the attention of the League Secretariat and began his transformation from national to international civil servant.