The 72-year-old cardinal is replacing Cardinal Carlo Furno, 82, former papal nuncio in Italy.
The title of archpriest of Roman basilicas, usually assigned to retired cardinals, entails administrative functions in a basilica and the presidency in liturgical celebrations.
John Paul II accepted Cardinal Law’s resignation as head of the Boston Archdiocese in December 2002 in the wake of the clerical sex abuse scandals.
Since his resignation, Cardinal Law has been chaplain at a Maryland convent, while keeping some of his posts in congregations and councils of the Roman Curia.
The Basilica of St. Mary Major was the fourth of the Roman patriarchal basilicas to be established. It was built between the years 432 and 440 on the ruins of a previous church to commemorate a miracle that occurred in 352.