The Trisagion (Greek: Τρισάγιον; 'Thrice Holy'), sometimes called by its opening line Agios O Theos, is a standard hymn of the Divine Liturgy in most of the Eastern Orthodox, Western Orthodox, Oriental Orthodox, and Eastern Catholic churches.

In churches which use the Byzantine Rite, the Trisagion is chanted immediately before the Prokeimenon and the Epistle reading. It is also included in a set of prayers named for it, called the Trisagion Prayers, which forms part of numerous services (the Hours, Vespers, Matins, and as part of the opening prayers for most services).

It is most prominent in the Latin Church for its use on Good Friday. It is also used in the Liturgy of the Hours and in some Catholic devotions.