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 the second signatory of the Proclamation of the Irish Republic. He was executed for his part in the Rising at age 33.