The Summa Theologiae or Summa Theologica (transl. 'Summary of Theology'), often referred to simply as the Summa, is the best-known work of Thomas Aquinas (1225–1274), a scholastic theologian and Doctor of the Church. It is a compendium of all of the main theological teachings of the Catholic Church, intended to be an instructional guide for theology students, including seminarians and the literate laity. Presenting the reasoning for almost all points of Christian theology in the West, topics of the Summa follow the following cycle: God; Creation, Man; Man's purpose; Christ; the Sacraments; and back to God.

Although unfinished, it is "one of the classics of the history of philosophy and one of the most influential works of Western literature." It remains Aquinas' "most perfect work, the fruit of his mature years, in which the thought of his whole life is condensed".

Throughout the Summa, Aquinas cites Christian, Muslim, Hebrew, and Pagan sources, including, but not limited to: Christian Sacred Scripture, Aristotle, Augustine of Hippo, Avicenna, Averroes, Al-Ghazali, Boethius, John of Damascus, Paul the Apostle, Pseudo-Dionysius, Maimonides, Anselm of Canterbury, Plato, Cicero, and John Scotus Eriugena.

The Summa is a more-structured and expanded version of Aquinas's earlier Summa contra Gentiles, though the two were written for different purposes. The Summa Theologiae intended to explain the Christian faith to beginning theology students, whereas the Summa contra Gentiles, to explain the Christian faith and defend it in hostile situations, with arguments adapted to the intended circumstances of its use, each article refuting a certain belief or a specific heresy.

Aquinas conceived the Summa specifically as a work suited to beginning students:

It was while teaching at the Santa Sabina studium provinciale—the forerunner of the Santa Maria sopra Minerva studium generale and College of Saint Thomas, which in the 20th century would become the Pontifical University of Saint Thomas Aquinas, Angelicum—that Aquinas began to compose the Summa. He completed the Prima Pars ('first part') in its entirety and circulated it in Italy before departing to take up his second regency as professor at the University of Paris (1269–1272).

Not only has the Summa Theologiae been one of the main intellectual inspirations for Thomistic philosophy, but it also had such a great influence on Dante Alighieri's Divine Comedy, that Dante's epic poem has been called "the Summa in verse". Even today, both in Western and Eastern Catholic Churches, and the mainstream original Protestant denominations (Anglicanism and Episcopalianism, Lutheranism, Methodism, and Presbyterianism), it is very common for the Summa Theologiae to be a major reference for those seeking ordination to the diaconate or priesthood, or for professed male or female religious life, or for laypersons studying philosophy and theology at the collegiate level.