Francisco Suárez SJ (/ˈswɑːrɛz/; 5 January 1548 – 25 September 1617) was a Spanish Jesuit priest, philosopher and theologian, one of the leading figures of the School of Salamanca movement. His work is considered a turning point in the history of second scholasticism, marking the transition from its Renaissance to its Baroque phases. According to Christopher Shields and Daniel Schwartz, "figures as distinct from one another in place, time, and philosophical orientation as Leibniz, Grotius, Pufendorf, Schopenhauer and Heidegger, all found reason to cite him as a source of inspiration and influence."

Francisco Suárez was born in Granada, Andalusia (southern Spain), on 5 January 1548. He was the youngest son of a noble family formed by the lawyer Gaspar Suárez de Toledo and his wife Antonia Vázquez de Utiel.

After 3 years of preliminary studies from age 10 onwards, in 1561 Suárez matriculated at the University of Salamanca, and studied law. In 1564, at age sixteen, Suárez entered the Society of Jesus in Salamanca and went through the two years of intense spiritual training under the guidance of Fr. Alonso Rodriguez. In August 1566, Suárez took his first vows as a Jesuit; he then began in October 1566 to study Theology at Salamanca. It seems he was not a promising student at first; in fact, he nearly gave up his matters of study after failing the entrance exam twice. After passing the exam at third attempt, though, things changed.

In 1570, with the completion of his course, Suárez began to teach Philosophy, first at Salamanca as a Scholastic tutor, and then as a professor in the Jesuit college at Segovia. He was ordained in March 1572 in Segovia. He continued to teach Philosophy in Segovia until, in September 1574, he moved to the Jesuit College in Valladolid to teach Theology, a subject he would then teach for the rest of his life. He taught in a succession of different places: Ávila (1575), Segovia (1575), Valladolid (1576) Rome (1580–85), Alcalá (1585–92) and Salamanca (1592–97). In 1597, he moved to Coimbra, some years after the accession of the Spanish (elder line) House of Habsburg to the Portuguese Throne, to take up the principal chair of Theology at the University of Coimbra. He remained there, aside from a brief time teaching at Rome, until his death in 1617.

He wrote on a wide variety of subjects, producing a vast amount of work (his complete works in Latin amount to twenty-six volumes). Suárez's writings include treatises on law, the relationship between Church and State, metaphysics, and theology. He is considered the godfather of International Law. His Disputationes metaphysicae (Metaphysical Disputations) were widely read in Europe during the 17th century and are considered by some scholars to be his most profound work.

Suárez was regarded during his lifetime as being the greatest living philosopher and theologian, and given the nickname Doctor Eximius et Pius ("Exceptional and Pious Doctor"); Pope Gregory XIII attended his first lecture in Rome. Pope Paul V invited him to refute the arguments of James I of England, and wished to retain him near his person, to profit by his knowledge. Philip II of Spain sent him to the University of Coimbra in order to give it prestige, and when Suárez visited the University of Barcelona, the doctors of the university went out to meet him wearing the insignia of their faculties.

After his death in Portugal (in either Lisbon or Coimbra) his reputation grew still greater, and he had a direct influence on such leading philosophers as Hugo Grotius, René Descartes, John Norris, and Gottfried Leibniz. His library was sent to Ethiopia in the mid-17th century, although many of the books went missing, with some of the books arriving in Portuguese Goa. He is buried in the Igreja de São Roque (formerly a Jesuit church) in Lisbon.

His most important philosophical achievements were in metaphysics and the philosophy of law. He adhered to a moderate form of Thomism and developed metaphysics as a systematic enquiry.