Seán Mac Diarmada (27 January 1883 – 12 May 1916), also known as Seán MacDermott, was an Irish republican political activist and revolutionary leader. He was one of the seven leaders of the Easter Rising of 1916, which he helped to organise as a member of the Military Committee of the Irish Republican Brotherhood (IRB) and was a signatory of the Proclamation of the Irish Republic. He was executed for his part in the Rising at age 33.

Brought up in rural County Leitrim, he was a member of many associations which promoted the cause of the Irish language, Gaelic revival and Irish nationalism in general, including the Gaelic League and (early in his career) the Irish Catholic fraternity the Ancient Order of Hibernians. He was national organiser for Sinn Féin, and later manager of the newspaper Irish Freedom, started in 1910 by Bulmer Hobson and others.