John O'Hart (1824–1902) was an Irish genealogist. He was born in Crossmolina, Co. Mayo, Ireland.
A committed Roman Catholic, O'Hart originally planned to become a Catholic priest but instead spent 2 years as a police officer. He was an Associate in Arts at the Queen's University of Belfast. He worked at the Commissioners of National Education during the years of the Great Irish Famine. He worked as a genealogist and took an interest in Irish history. He was an Irish nationalist. He died in 1902 in Clontarf near Dublin, at the age of 78.
O'Hart's 800-page The Irish and Anglo-Irish landed gentry (Dublin 1884) was reprinted in 1969, with an introduction by Edward MacLysaght, the first Chief Herald of Ireland. Another work, Irish pedigrees; or, The origin and stem of the Irish nation, first published in 1876, has come out in several subsequent editions.
To complete his genealogies he used the writings of Cú Choigcríche Ó Cléirigh, Dubhaltach Mac Fhirbhisigh and O'Farrell, along with the Annals of the Four Masters, for the medieval pedigrees. He used the works of Bernard Burke, John Collins and others to extend his genealogies past the 17th century.