Joseph Calasanz, Spanish priest and founder of Piarists (died 1648)

Joseph Calasanz, also known as Joseph Calasanctius and Iosephus 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 canonization.

The Piarists, officially named the Order of Poor Clerics Regular of the Mother of God of the Pious Schools, abbreviated SchP, is a religious order of clerics regular of the Catholic Church founded in 1617 by Spanish priest Joseph Calasanz. It is the oldest religious order dedicated to education, and the main occupation of the Piarist fathers is teaching children and youth, the primary goal being to provide free education for poor children. The Piarist practice was to become a model for numerous later Catholic societies devoted to teaching, while some state-supported public school systems in Europe also followed their example. The Piarists have had a considerable success in the education of physically or mentally disabled persons. Notable individuals who have taught at Piarist schools include Pope Pius IX, Goya, Schubert, Gregor Mendel, Tadeusz Kościuszko, and Victor Hugo.

September 11 is the 254th day of the year in the Gregorian calendar; 111 days remain until the end of the year.