John Marcus O'Sullivan (18 February 1881 – 9 February 1948) was an Irish politician, cabinet minister and academic.
O'Sullivan was born in Killarney, County Kerry in 1881. He was educated at St. Brendan's, Killarney; Clongowes Wood College, County Kildare; University College Dublin (UCD); University of Bonn and Heidelberg University, where he was awarded a PhD. He was appointed to the Chair of Modern History at UCD in 1910.
He was first elected to Dáil Éireann in 1923 as a Cumann na nGaedheal Teachta Dála (TD) for the Kerry North constituency. He served as Parliamentary Secretary to the Minister for Finance from 1924–26. He was appointed to the Cabinet in 1926, serving under W. T. Cosgrave as Minister for Education. In 1926 a report from the Second National Programme Conference was presented to him as the Minister for Education. He accepted all proposals stated in the report to be recommended as a national curriculum. His major ministerial achievement was the Vocational Education Act 1930. He served on the Irish delegation to the League of Nations, in 1924 and from 1928–30. He was re-elected at every election until 1943 when he lost his Dáil seat. He subsequently retired from politics.
O'Sullivan died in 1948, five years after retiring from politics.