Sean MacDermott (February 28, 1884 – May 12, 1916) was one of the leaders of the Easter Rising of 1916 in Ireland.

He was born John MacDermott in County Leitrim in 1884, though later in life he adopted the Irish form of his name: Sean MacDiarmada¹. In 1908 he moved to Dublin, by which time he was already involved in several separatist organizations, including Sinn Fein, and the Irish Republican Brotherhood (IRB). He was soon promoted to the Supreme Council of the IRB upon the recommendation of a senior member who was being removed for excessive drunkeness, as part of the ongoing reorganization, and he eventually was elected secretary.

In 1910 he became manager of the radical newspaper "Irish Freedom," which he founded along with Bulmer Hobson and Denis McCullough. He also became a national organizer for the IRB, and was taken under the wing of veteran Fenian Tom Clarke. Indeed over the year the two became nearly inseparable. Shortly thereafter MacDermott was stricken with polio and forced to walk with a cane.

In November, 1913 MacDermott was one of the original members of the Irish Volunteers, and continued to work effortlessly to bring that organization under IRB control. MacDermott proved himself to be one of the most radical and Machiavellian members until his death. When his close friend Bulmer Hobson voted to allow the Irish Parliamentary leader John Redmond to take control of the Volunteers, MacDermott never spoke to him again.

In May, 1915, MacDermott was arrested in Tuam, County Galway, under the Defense of the Realm Act for giving a speech against enlisting into the British Army. He was released in September, where upon he joined the secret Military Committee of the IRB, which was responsible for planning the rising. Indeed it was MacDermott and Clarke who were most responsible for it.

Being somewhat crippled, MacDermott took little part in the fighting of Easter week, but was stationed at the headquarters in the General Post Office. Following the surrender, he nearly escaped execution by blending in with the large body of prisoners, but was eventually recognized and summarily executed by firing squad on May 12.

¹ The form of his name used here, "Sean MacDermott", is a hybrid of the English and Gaelic forms of his name, and for that reason may be considered inaccurate. However, it is also probably the most common form used, so this article has been listed under it. The other common forms should all redirect here.