Compline (/ˈkɒmplɪn/ KOM-plin), also known as Complin, Night Prayer, or the Prayers at the End of the Day, is the final prayer liturgy (or office) of the day in the Christian tradition of canonical hours, which are prayed at fixed prayer times.

The English word is derived from the Latin completorium, as compline is the completion of the waking day. The word was first used in this sense about the beginning of the 6th century in the Rule of Saint Benedict (Regula Benedicti; hereafter, RB), in Chapters 16, 17, 18, and 42, and he uses the verb compleo to signify compline: "Omnes ergo in unum positi compleant" ("All having assembled in one place, let them say compline"); "et exeuntes a completorio" ("and, after going out from compline")… (RB, Chap. 42).

Compline liturgies are a part of Catholic, Anglican, Lutheran, Oriental Orthodox, Eastern Orthodox, and certain other Christian liturgical traditions.

In Western Christianity, Compline tends to be a contemplative office that emphasizes spiritual peace. In most monasteries it is the custom to begin the "Great Silence" after compline, during which the whole community, including guests, observes silence throughout the night until after the Terce the next day. Compline comprises the final office in the Liturgy of the Hours.

From the time of the early Church, the practice of seven fixed prayer times has been taught; in Apostolic Tradition, Hippolytus instructed Christians to pray seven times a day "on rising, at the lighting of the evening lamp, at bedtime, at midnight" and "the third, sixth and ninth hours of the day, being hours associated with Christ's Passion."

The origin of compline has given rise to considerable discussion among liturgists. In the past, general opinion ascribed the origin of this liturgical hour to St. Benedict, at the beginning of the 6th century. But Jules Pargoire and A. Vandepitte trace its source to Saint Basil. Vandepitte states that it was not in Cæsarea in 375, but in his retreat in Pontus (358–362), that Basil established compline, which hour did not exist prior to his time, that is, until shortly after the middle of the 4th century. François Plaine [fr] also traced the source of compline back to the 4th century, finding mention of it in a passage in Eusebius and in another in St. Ambrose, and also in John Cassian. These texts bear witness to the private custom of saying a prayer before retiring to rest. If this was not the canonical hour of compline, it was certainly a preliminary step towards it. The same writers reject the opinion of Paulin Ladeuze and Jean-Martial Besse [fr] who believe that compline had a place in the Rule of St. Pachomius, which would mean that it originated still earlier in the 4th century.

The Catholic Encyclopedia argues that, if St. Basil instituted and organized the hour of compline for the East, as St. Benedict did for the West, there existed as early as the days of St. Cyprian and Clement of Alexandria the custom of reciting a prayer before sleep, and that this might be taken as the original source of compline.

It is generally thought that the Benedictine form of compline is the earliest western order, although some scholars, such as Plaine, have maintained that the hour of compline as found in the Roman Breviary at his time, antedated the Benedictine Office. These debates apart, Benedict's arrangement probably invested the hour of compline with the liturgical character and arrangement which were preserved in the Benedictine Order, and largely adopted by the Roman Church. The original form of the Benedictine Office, lacking even an antiphon for the psalms, is much simpler than its Roman counterpart, resembling more closely the Minor Hours of the day.