John O'Hart (1824–1902) was an Irish genealogist. He was born in Crossmolina, County Mayo, Ireland. A committed Roman Catholic and Irish nationalist, O'Hart had originally planned to become a priest but instead spent two years as a police officer. He was an Associate in Arts at the Queen's University, Belfast. He worked at the Commissioners of National Education during the years of the Great Famine. He worked as a genealogist and took an interest in Irish history. He died in 1902 in Clontarf near Dublin, at the age of 78.