Joseph Calasanz Sch.P. (Spanish: José de Calasanz; Italian: Giuseppe Calasanzio; September 11, 1557 – August 25, 1648), also known as Joseph Calasanctius and Josephus a Matre Dei, was a Spanish Catholic priest, educator and the founder of the Pious Schools, which provided free education to poor boys. For this purpose he founded the religious order that ran them, commonly known as the Piarists. He became a close friend of the renowned astronomer Galileo Galilei. Joseph is honored as a saint by the Catholic Church, following his 1767 papal canonisation.

Calasanz was born in Peralta de Calasanz (before de la Sal) in the Kingdom of Aragon, on September 11, 1557, the youngest in a family of eight children, and the second son. His parents were Pedro de Calasanz y de Mur, an infanzón (minor nobleman) and town mayor, and Pedro's wife María Gastón y de Sala. Joseph had two sisters, Marta and Cristina. In his youth, he was nearly two meters tall. His parents gave him a good education at home and then at the elementary school of Peralta. In 1569, he was sent for classical studies to a college in Estadilla run by the friars of the Trinitarian Order. While there, at the age of 14, he determined that he wanted to become a priest. In this, however, he met with no support from his parents.

For his higher studies, Calasanz took up philosophy and law at the University of Lleida, where he was awarded the degree of Doctor of Laws cum laude. He then began theology studies at the University of Valencia and at the Complutense University, which at the time was still located at its original site in Alcalá de Henares.

Joseph's mother and brother having died, his father wanted him to marry and carry on the family. But a sickness in 1582 soon brought Joseph to the brink of the grave, which caused his father to relent. When he recovered, the Bishop of Urgel, Hugo Ambrosio de Moncada, ordained him as a priest on December 17, 1583.

During his ecclesiastical career in Spain, Calasanz held various offices in his native region. He began his ministry in the diocese of Albarracín, where Bishop Gaspar Juan de la Figuera appointed him his theologian, confessor, synodal examiner, and procurator. When the bishop was transferred to Lerida, Calasanz followed him to the new diocese. During that period, he spent several years in La Seu d'Urgell. As secretary of the cathedral chapter, Calasanz had broad administrative responsibilities. In Claverol, a distant village to the northeast of Lerida, he established a foundation that distributed food to the poor.

In October 1585, Bishop de la Figuera was sent as apostolic visitor to the Abbey of Montserrat and Calasanz accompanied him as his secretary. The bishop died the following year and Calasanz left, though urgently requested to remain. He hurried to Peralta de Calasanz to attend the death of his father. He was then called by the Bishop of Urgel to act as vicar general for the district of Tremp.

In 1592, at 35, Calasanz moved to Rome. He hoped to further his ecclesiastical career and secure some kind of benefice. He lived there for most of his remaining 56 years. In Rome, Calasanz found a protector in Cardinal Marcoantonio Colonna, who chose him as his theologian and, once he had learned to express himself in Italian, entrusted him with the spiritual direction of his household. The city of Rome offered many opportunities for works of charity, especially for the instruction of neglected and homeless children, many of whom had lost their parents. Joseph joined the Confraternity of Christian Doctrine and gathered boys from the streets for religious instruction and other schooling. However, this initiative was not well received by teachers at the school. Being poorly paid, they refused to accept the additional labor without remuneration.

The pastor of the Church of Santa Dorotea in Trastevere, Anthony Brendani, offered Joseph two rooms just off of the parish sacristy and promised assistance in teaching, and when two other priests promised similar help, Calasanz, in 1597 (November 27), opened what was thought to be the first free public school in Europe.