James Dillon, 3rd Earl of Roscommon (c. 1605 – 1649) was an Irish peer of the seventeenth century. He was one of the ten persons named in the Act for the Settlement of Ireland 1652 as leaders of the Royalist forces in Ireland, thus rendering their estates liable to forfeiture, although he had by then been dead for three years.
He was a Protestant nobleman, the eldest son of Robert Dillon, 2nd Earl of Roscommon and his first wife Margaret Barry, daughter of David de Barry, 5th Viscount Buttevant and his first wife Ellen Roche. His family were traditionally members of the Roman Catholic faith, but James was converted to the Church of Ireland by James Ussher, Archbishop of Armagh.
He married Elizabeth Wentworth, daughter of Sir William Wentworth and Anne Atkins, and sister of Thomas Wentworth, 1st Earl of Strafford, the formidable and (for a time), all-powerful Lord Deputy of Ireland, a marriage which was clearly intended to strengthen English rule in Ireland through family alliances between leading English and Anglo-Irish families.Wentworth Dillon, 5th Earl of Roscommon and a poet of repute, was their son.Carey Dillon, 5th Earl of Roscommon, the "Colonel Dillon" of the Diary of Samuel Pepys, was his younger half-brother.
He died at Limerick in October 1649, from an accidental fall downstairs. According to legend his son knew of his death at the moment it happened, although the official news of the death did not reach him until two weeks later.