priest, teacher, patron saint of Roman Catholic schools, and founder of the Ordo Clericorum Regularium Pauperum Matris Dei Scholarum Piarum (Order of Poor Clerks Regular of the Mother of God of the Pious Schools), popularly called Piarists. The Piarists are a teaching order that, in addition to the usual vows of poverty, chastity, and obedience, practiced a fourth vow: the special care of youth.

Of aristocratic birth, Calasanz was educated at the Spanish universities of Estadilla, Lérida, and Valencia, was ordained priest in 1583, and subsequently became vicar general of Tremp. Later he relinquished much of his inheritance, resigned his vicariate, and went to Rome (1592), where he laboured for the education of poor children. In November 1597 he opened Europe's first free school for poor children.

After other priests joined him, they eventually expanded their headquarters, forming a kind of community life. Increasing student enrollment necessitated a move in 1602, and, after a crippling accident, Joseph was financially assisted by popes Clement VIII and Paul V, who in 1617 recognized his community as a religious congregation. In 1621 the congregation, which was spreading throughout Italy, became papally approved as a religious order, with Joseph as superior.

Mario Sozzi, a priest who was admitted to the Piarists in 1630, eventually caused, out of jealousy, dissension that led to Joseph's unwarranted arrest. An ensuing internal revolt ruptured the order; in 1643 Sozzi died, being succeeded by an equally villainous subordinate, Fr. Cherubini, and Pope Urban VIII quashed the generalate. Joseph, then 86 years old, was tried by the Holy See. Pope Innocent X reinstated him, but in 1646, after further internal disruption, the order was papally reduced to a mere society in which each priest was subject to his bishop. Joseph's hope for the Piarists' complete restoration was not fulfilled until after his death.

Joseph was declared patron of all Christian schools by Pope Pius XII in 1948.