Thomas Aquinas, OP (; Italian: Tommaso d'Aquino, lit. 'Thomas of Aquino'; 1225 – 7 March 1274) was an Italian Dominican friar and priest, an influential philosopher and theologian, and a jurist in the tradition of scholasticism from the county of Aquino in the Kingdom of Sicily, Italy; he is known within the tradition as the Doctor Angelicus, the Doctor Communis, and the Doctor Universalis. In 1999, John Paul II added a new title to these traditional ones: Doctor Humanitatis.

Thomas was a prominent proponent of natural theology and the father of a school of thought (encompassing both theology and philosophy) known as Thomism. He argued that God is the source of the light of natural reason and the light of faith. He has been described as "the most influential thinker of the medieval period" and "the greatest of the medieval philosopher-theologians". His ideas, unlike many currents in the Catholic Church of the time, embraced several ideas put forward by Aristotle—whom he called "the Philosopher"—and attempted to synthesize Aristotelian philosophy with the principles of Christianity.

His best-known works are the Disputed Questions on Truth (1256–1259), the Summa contra Gentiles (1259–1265), and the unfinished Summa Theologica, or Summa Theologiae (1265–1274). His commentaries on Scripture and on Aristotle also form an important part of his body of work. Furthermore, Thomas is distinguished for his eucharistic hymns, which form a part of the church's liturgy. The Catholic Church honors Thomas Aquinas as a saint and regards him as the model teacher for those studying for the priesthood, and indeed the highest expression of both natural reason and speculative theology. In modern times, under papal directives, the study of his works was long used as a core of the required program of study for those seeking ordination as priests or deacons, as well as for those in religious formation and for other students of the sacred disciplines (philosophy, Catholic theology, church history, liturgy, and canon law).

As a Doctor of the Church, Thomas Aquinas is considered one of the Catholic Church's greatest theologians and philosophers. Pope Benedict XV declared: "This (Dominican) Order ... acquired new luster when the Church declared the teaching of Thomas to be her own and that Doctor, honored with the special praises of the Pontiffs, the master and patron of Catholic schools."