Seán Lester (28 September 1888 – 13 June 1959) was an Irish diplomat who was the last secretary-general of the League of Nations from 31 August 1940 to 18 April 1946.

Although 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.

He worked as a journalist for the North Down Herald and a number of other northern papers before he moved to Dublin, where he found a job at the Freeman's Journal.

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.

When Peru and Colombia had a dispute over a town in the headwaters of the Amazon, Lester presided over the committee that found an equitable solution.

Lester repeatedly protested to the German government over its persecution and discrimination of Jews and warned the League of the looming disaster for Europe.