Thomas MacDonagh (February 1, 1878 - May 3, 1916) was an Irish nationalist, poet, and a leader of the 1916 Easter Rising. MacDonagh was born in Cloughjordan, County Tipperary in 1878. Throughout his life he had a keen interest in Irish heritage and language. He moved to Dublin where he joined the Gaelic League, soon establishing strong friendships with such men as Eoin MacNeill and Patrick Pearse.

His friendship with Pearse and his love of Gaelic led him to join the staff of Pearse's bilingual St. Enda's School upon its establishment in 1908, taking the role of teacher and Assistant Headmaster. Though MacDonagh was essential to the school's early success, he soon moved on to take the position of Professor of English at the National University . MacDonagh remained devoted to the Irish language, and in 1910 he became tutor to a younger member of the Gaelic League, Joseph Plunkett. The two were both poets with an interest in the Irish Theatre, and formed a lifelong friendship. In 1912 he married Muriel Gifford; their son Donagh was born later that year.

In 1913 both MacDonagh and Plunkett attended the inaugural meeting of the Irish Volunteers and were placed on its Provisional Committee. He was later appointed commandant of Dublin's 2nd battalion, and eventually made commandant of the entire Dublin Brigade. Though originally more of a constitutionalist, through his dealings with men such as Pearse, Plunkett, and Sean MacDermott, MacDonagh developed stronger republican beliefs, joining the Irish Republican Brotherhood (IRB), probably during the summer of 1915. Around this time Tom Clarke asked him to plan the grandiose funeral of Jeremiah O'Donovon Rossa , which was a resounding success.

Though credited as one of the Easter Rising's seven leaders, MacDonagh was a late addition to that group. He didn't join the secret Military Council that planned the rising until April 1916, weeks before the rising took place. The reason for his admittance at such a late date is uncertain. Still a relative newcomer to the IRB, men such as Clarke may have been hesitant to elevate him to such a high position too soon, which begs the question why admit him at all. His close ties to Pearse and Plunkett may have been the cause, as well as his position as commandant of the Dublin Brigade (though his position as such would later be superseded by James Connolly as commandant-general of the Dublin division). Nevertheless, MacDonagh was a signatory of the Easter Proclamation.

During the rising, MacDonagh's battalion was stationed at the massive complex of Jacob's Biscuit Factory. On the way to this destination the battalion encountered the veteran Fenian John MacBride, who on the spot joined the battalion as second-in-command, and in fact took over much of the command throughout Easter week. As it was, despite MacDonagh's rank and the fact that he commanded one of the strongest battalions, they saw little fighting, as the British Army easily circumvented the factory as they established positions in central Dublin. MacDonagh received the order to surrender on April 30, though his entire battalion was fully prepared to continue the engagement. Following the surrender, MacDonagh was court martialled, and executed by firing squad on May 3.

MacDonagh was generally credited with being one of the most gregarious and personable of the rising's leaders.

His son Donagh MacDonagh went on to become a prominent poet, playwright, songwriter, and judge before his death in 1968.