A Requiem or Requiem Mass, also known as Mass for the dead (Latin: Missa pro defunctis) or Mass of the dead (Latin: Missa defunctorum), is a Mass of the Catholic Church offered for the repose of the soul or souls of one or more deceased persons, using a particular form of the Roman Missal. It is usually celebrated in the context of a funeral (where in some countries it is often called a Funeral Mass).

Musical settings of the propers of the Requiem Mass are also called Requiems, and the term has subsequently been applied to other musical compositions associated with death, dying, and mourning, even when they lack religious or liturgical relevance.

The term is also used for similar ceremonies outside the Roman Catholic Church, especially in Western Rite Orthodox Christianity, the Anglo-Catholic tradition of Anglicanism, and in certain Lutheran churches. A comparable service, with a wholly different ritual form and texts, exists in the Eastern Orthodox and Eastern Catholic churches as well as some Methodist churches.

The Mass and its settings draw their name from the introit of the liturgy, which begins with the words Requiem aeternam dona eis, Domine (Latin for "Eternal rest grant them, O Lord"), which is cited from 2 Esdras—requiem is the accusative singular form of the Latin noun requies, "rest, repose". The Roman Missal as revised in 1970 employs this phrase as the first entrance antiphon among the formulas for Masses for the dead, and it remains in use to this day.