Known by various names, this Franciscan Province at one time comprised the territory of all the states in the vast basin of the Rio de la Plata and its great tributaries: the Rio de la Plata and Tucumán colonies (present Argentina), Paraguay, and Uruguay—and in the beginning also the southernmost part of Brazil.

Though it was the last of the Franciscan Provinces to be established in Spanish South America during colonial times, it was founded as the independent Custody of Paraguay-Plata as early as 1538. In 1565 another Custody, that of St. George of Tucumán, was founded by Franciscans of the Peruvian Province (see The Americas, II, 83–90). These two Custodies were united into one in 1575, but separated again into two in 1597, both remaining dependent on the Province in Peru. Fifteen years later, the General Chapter held in Rome on July 12, 1612, once more joined the two Custodies to form the Province of Rio de la Plata.