Mary Teresa Hayden (1862–12 July 1942) was an Irish historian, Irish-language activist and campaigner for women's causes.
Mary Hayden was educated initially at the Dominican College, Eccles Street and then at Alexandra College in Dublin. She attended the Royal University of Ireland where she graduated with a BA (1885) and an MA (1887) in Modern Languages. With Agnes O'Farrelly she campaigned for women's rights in the university.
A campaigner for gender equality and noted as a public speaker, she was a prominent member of the Irish Women's Suffrage and Local Government Association. She was a member of the Gaelic League and friends with Padraig Pearse. However, she opposed violence and disapproved of the 1916 Rising.
- A Short History of the Irish People by Mary Hayden and George Moonan(1921)
- Diaries of Mary Hayden 1878–1903 edited by Conan Kennedy (National Library of Ireland/Morrigan, 2008)
- The Mysterious Mary H. : A Selection of Writings by Mary Hayden (1862–1942) (Morrigan Books, 2008).
- "Mary Hayden (1862–1942)". Scoilnet.ie. Retrieved 30 September 2013.
- Ten Dublin women by Medb Ruane, Dublin, 1991.