The introit (Latin: introitus, "entrance") is part of the opening of the celebration of the Mass. Specifically, it refers to that part of the Tridentine Mass, though the term is used in other orders of the Mass to label the antiphon that is spoken or sung at the beginning of the celebration. It is one of the "propers" of the Mass, that is, a part that changes over the liturgical year.

Most introits are taken from Psalms, though there exist those from Proverbs or Wisdom. Generally they follow the same structure: two to four lines of scripture related to the "theme" of the feastday or celebration. Most often the choice of scripture passage has something in common with the liturgical readings that will be featured later in that Mass.

The introit can be either sung or spoken, depending on the formality of the Mass as well as the preferences of the priest and his congregation. Not all Masses have introits; traditionally the service of Holy Saturday in the Roman Catholic rite does not.

The practice evolved from the singing of a psalm as the priest and ministers approached the altar, sung verses having been part of the celebration of the Mass since earliest times. The Liber Pontificalis claims that the introit originated by the request of Celestine I, but it was in the reign of Gregory I that the familiar form emerged, and Gregory is credited with "composing" many introits himself.

In many cases the introit also serves another purpose: it gives a name to a particular Mass, based on the first word or phrase of the introit. Since the introit for a Mass is different for each day of the Liturgical Calendar (sometimes multiple Masses even exist for a single day, such as Christmas), and unique Masses exist for special functions, it provides a key for determining which order of the Mass is to be performed. This is why a funeral Mass is called a Requiem and the second Mass of the Feast of the Sacred Heart is called the Exordium, for example. Even though Latin is no longer used widely in the celebration of the Mass, the traditional names remain for purposes of organization of the calendar.

An interesting side effect of this is that unnamed feast days acquire the names of their respective Masses. As an unusual example, the first Sunday after Easter Sunday, though having no official liturgical name, is colloquially labelled "Quasimodo Sunday" as the first phrase of the introit is "Quasi modo geniti infantes...".