O'Dowd (), is an Irish Gaelic clan based most prominently in what is today County Mayo and County Sligo. The clan name originated in the 9th century as a derivative of its founder Dubda mac Connmhach. They descend in the paternal line from the Connachta's Uí Fiachrach. The immediate progenitors of the O'Dowd were Kings of Connacht during the 7th and 8th centuries in the form of Dúnchad Muirisci, Indrechtach mac Dúnchado, Ailill Medraige mac Indrechtaig and Donn Cothaid mac Cathail, before losing ground to their rivals the Uí Briúin.