Niccolò Gaddi (1499–1552) was an Italian Roman Catholic cardinal.
Niccolò Gaddi was born in Florence in 1499, the son of Taddeo Gaddi and Antonia Altoviti. He was a direct descendant of medieval painter Taddeo Gaddi. He was the uncle of Cardinal Taddeo Gaddi and a relative of Catherine de' Medici.
He began his career in Rome as a cleric in the Apostolic Camera. He then became an abbreviator of Apostolic Letters.
On October 16, 1521, he was elected Bishop of Fermo. He was never consecrated as a bishop and he resigned the administration of the see sometime before December 5, 1544.
Pope Clement VII made him a cardinal deacon in the consistory of May 3, 1527. He received the red hat and the deaconry of San Teodoro on the same day.
During the Sack of Rome (1527), he was assigned as a hostage for Pope Clement VII.Imperial troops held him prisoner in the fortress in Naples for a long time.
He was the administrator of the metropolitan see of Cosenza from January 31, 1528 until June 21, 1535. In 1533, he was named cardinal protector of the Kingdom of France. He was administrator of the see of Sarlat from December 12, 1533 until July 3, 1545.