Thomas Aquinas OP (/əˈkwaɪnəs/ ^ⓘ ə-KWY-nəs; Italian: Tommaso d'Aquino, lit. 'Thomas of Aquino'; c. 1225 – 7 March 1274) was an Italian Dominican friar and priest, the foremost Scholastic thinker, as well as one of the most influential philosophers and theologians in the Western tradition. A Doctor of the Church, he was from the county of Aquino in the Kingdom of Sicily.

Thomas was a 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 embraced several ideas put forward by Aristotle and attempted to synthesize Aristotelian philosophy with the principles of Christianity. He has been described as "the most influential thinker of the medieval period" and "the greatest of the medieval philosopher-theologians". Thomas Aquinas's philosophy influenced modern virtue ethics, aesthetics, and cognitive theory. He has been criticized, notably by Bertrand Russell, for seeking to justify conclusions already dictated by faith rather than follow reason independently.

Thomas's best-known works are the unfinished Summa Theologica, or Summa Theologiae (1265–1274), the Disputed Questions on Truth (1256–1259) and the Summa contra Gentiles (1259–1265). His commentaries on Christian Scripture and on Aristotle also form an important part of his body of work. He is also notable for his Eucharistic hymns, which form a part of the Church's liturgy.

As a Doctor of the Church, Thomas is considered one of the Catholic Church's greatest theologians and philosophers. He is known in Catholic theology as the Doctor Angelicus ("Angelic Doctor", with the title "doctor" meaning "teacher"), and the Doctor Communis ("Universal Doctor"). In 1999 Pope John Paul II added a new title to these traditional ones: Doctor Humanitatis ("Doctor of Humanity/Humaneness").

Thomas Aquinas was most likely born in the family castle of Roccasecca, near Aquino, controlled at that time by the Kingdom of Sicily (in present-day Lazio, Italy), c. 1225. He was born to the most powerful branch of the family, and his father, Landulf VI of Aquino, Lord of Roccasecca, was a man of means. As a knight in the service of Frederick II, Holy Roman Emperor, Landulf of Aquino held the title miles. Thomas's mother, Theodora Galluccio, Countess of Teano, belonged to the Rossi branch of the Neapolitan Caracciolo family. Landulf's brother Sinibald was abbot of Monte Cassino, the oldest Benedictine monastery. He was the youngest of four sons and had two sisters. While the rest of the family's sons pursued military careers, the family intended for Thomas to follow his uncle into the abbacy; this would have been a normal career path for a younger son of Southern Italian nobility.

At the age of five, Thomas began his early education at Monte Cassino. After the military conflict between Frederick II and Pope Gregory IX spilt into the abbey in early 1239, Landulf and Theodora had Thomas enrolled at the studium generale (university) established by Frederick in Naples. There, his teacher in arithmetic, geometry, astronomy, and music was Petrus de Ibernia. According to his biographer Guglielmo Tocco, Martin of Dacia was his teacher of grammar and logic. It was at this university that Thomas was presumably introduced to Aristotle, Averroes and Maimonides, all of whom would influence his theological philosophy. Peter of Ireland was teaching the recently translated works of Aristotle as commented on by the Spanish-Arabic philosopher Averroes. During his study at Naples, Thomas also came under the influence of John of St. Julian, a Dominican preacher in Naples, who was part of the active effort by the Dominican Order to recruit devout followers.

At the age of nineteen, Thomas resolved to join the Dominican Order. His change of heart, however, did not please his family. In an attempt to prevent Theodora's interference in Thomas's choice, the Dominicans arranged to move Thomas to Rome, and from Rome, to Paris. However, while on his journey to Rome, per Theodora's instructions, his brothers seized him as he was drinking from a spring and took him back to his parents at the castle of Monte San Giovanni Campano.

Thomas was held prisoner for almost one year in the family castles at Monte San Giovanni and Roccasecca in an attempt to prevent him from assuming the Dominican habit and to push him into renouncing his new aspiration. Political concerns prevented the Pope from ordering Thomas's release, which had the effect of extending Thomas's detention. Thomas passed this time of trial tutoring his sisters and communicating with members of the Dominican Order.