Thomas Fleming (1593–1665) was an Irish Franciscan and Roman Catholic Archbishop of Dublin; he was heir to the title Baron Slane, but renounced it.
He was the eldest son of Christopher Fleming,12th Baron Slane and Eleanor, daughter of Patrick Barnewall: on his father's death in 1625 he succeeded as 13th Baron, but renounced the title in favour of his brother William, 14th Baron. He studied at the Franciscan College at Leuven, became a priest of the Franciscan Order, and after finishing his studies continued at the Catholic University of Leuven for a number of years as professor. In October 1623, he was appointed by Pope Urban VIII to Dublin as successor of Archbishop Matthews.
His appointment gave great offense to opponents of the religious orders, and a bitter onslaught was begun against the new archbishop by the priest Paul Harris, in his Olfactorium and other brochures. Archbishop Fleming convened and presided at a provincial synod of the province of Dublin in 1640. Strafford, the all powerful Lord Deputy of Ireland, had no sympathy for Irish Catholics, but did not favour persecution: like the Duke of Ormonde later he was prepared to tacitly recognise the hierarchy, and even granted Fleming a personal audience, although he described him afterwards in rather unflattering terms. When the Irish Rebellion of 1641 broke out (1641–1642) the archbishop, though rather a man of peace, felt constrained to take sides with the Confederates and despatched a procurator to represent him at the synod of the clergy held in Kilkenny (May 1642). Later on, when the general assembly was convoked at Kilkenny for October, the archbishop resolved to attend personally and take part in the deliberations.
As might be expected from his antecedents, and especially from his connection with the Anglo-Irish nobility of the Pale, he was opposed to the "thorough" policy of the Old Irish, and wished for peace at all costs. In 1643 he was one of the prelates who signed the commission empowering representatives of the Confederates to treat with James Butler, 1st Duke of Ormonde for a cessation of hostilities. He also opposed Pierfrancesco Scarampi and Giovanni Battista Rinuccini, the latter of whom was strongly identified with the Old-Irish party.