Gregorian chant is the central tradition of Western plainchant, a form of monophonic, unaccompanied sacred song in Latin (and occasionally Greek) of the Roman Catholic Church. Gregorian chant developed mainly in western and central Europe during the 9th and 10th centuries, with later additions and redactions. Although popular legend credits Pope Gregory I with inventing Gregorian chant, scholars believe that he only ordered a compilation of melodies throughout the whole Christian world, after having instructed his emissaries in the Schola Cantorum, where the neumatical notation was perfected, with the result of most of those melodies being a later Carolingian synthesis of the Old Roman chant and Gallican chant.

Gregorian chants were organized initially into four, then eight, and finally 12 modes. Typical melodic features include a characteristic ambitus, and also characteristic intervallic patterns relative to a referential mode final, incipits and cadences, the use of reciting tones at a particular distance from the final, around which the other notes of the melody revolve, and a vocabulary of musical motifs woven together through a process called centonization to create families of related chants. The scale patterns are organized against a background pattern formed of conjunct and disjunct tetrachords, producing a larger pitch system called the gamut. The chants can be sung by using six-note patterns called hexachords. Gregorian melodies are traditionally written using neumes, an early form of musical notation from which the modern four-line and five-line staff developed. Multi-voice elaborations of Gregorian chant, known as organum, were an early stage in the development of Western polyphony.

Gregorian chant was traditionally sung by choirs of men and boys in churches, or by women and men of religious orders in their chapels. It is the music of the Roman Rite, performed in the Mass and the monastic Office. Although Gregorian chant supplanted or marginalized the other indigenous plainchant traditions of the Christian West to become the official music of the Christian liturgy, Ambrosian chant still continues in use in Milan, and there are musicologists exploring both that and the Mozarabic chant of Christian Spain. Although Gregorian chant is no longer obligatory, the Roman Catholic Church still officially considers it the music most suitable for worship.

Singing has been part of the Christian liturgy since the earliest days of the Church. It is widely accepted that the psalmody of ancient Jewish worship significantly influenced and contributed to early Christian ritual and chant. Christians read Scriptures and sang chants, as their Jewish predecessors had done. Although new Christian liturgy was developed, the source of much of this Christian liturgy was Jewish psalmody. The source materials for newly emergent Christian chants were originally transmitted by Jews in sung form. Early Christian rites also incorporated elements of Jewish worship that survived in later chant tradition. Canonical hours have their roots in Jewish prayer hours. "Amen" and "alleluia" come from Hebrew, and the threefold "sanctus" derives from the threefold "kadosh" of the Kedushah.

The New Testament mentions singing hymns during the Last Supper: "When they had sung the hymn, they went out to the Mount of Olives" (Matthew 26.30). Other ancient witnesses such as Pope Clement I, Tertullian, St. Athanasius, and Egeria confirm the practice, although in poetic or obscure ways that shed little light on how music sounded during this period.

Musical elements that would later be used in the Roman Rite began to appear in the 3rd century. The Apostolic Tradition, attributed to the theologian Hippolytus, attests the singing of Hallel (Jewish) psalms with Alleluia as the refrain in early Christian agape feasts. Chants of the Office, sung during the canonical hours, have their roots in the early 4th century, when desert monks following St. Anthony introduced the practice of continuous psalmody, singing the complete cycle of 150 psalms each week. Around 375, antiphonal psalmody became popular in the Christian East; in 386, St. Ambrose introduced this practice to the West. In the fifth century, a singing school, the Schola Cantorum, was founded at Rome to provide training in church musicianship.

Scholars are still debating how plainchant developed during the 5th through the 9th centuries, as information from this period is scarce. Around 410, St. Augustine described the responsorial singing of a Gradual psalm at Mass. At c. 520, Benedict of Nursia established what is called the rule of St. Benedict, in which the protocol of the Divine Office for monastic use was laid down. Around 678, Roman chant was taught at York. Distinctive regional traditions of Western plainchant arose during this period, notably in the British Isles (Celtic chant), Spain (Mozarabic), Gaul (Gallican), and Italy (Old Roman, Ambrosian and Beneventan). These traditions may have evolved from a hypothetical year-round repertory of 5th-century plainchant after the western Roman Empire collapsed.

John the Deacon, biographer (c. 872) of Pope Gregory I, modestly claimed that the saint "compiled a patchwork antiphonary", unsurprisingly, given his considerable work with liturgical development. He reorganized the Schola Cantorum and established a more uniform standard in church services, gathering chants from among the regional traditions as widely as he could manage. Of those, he retained what he could, revised where necessary, and assigned particular chants to the various services. According to Donald Jay Grout, his goal was to organize the bodies of chants from diverse traditions into a uniform and orderly whole for use by the entire western region of the Church. His renowned love for music was recorded only 34 years after his death; the epitaph of Honorius testified that comparison to Gregory was already considered the highest praise for a music-loving pope. While later legends magnified his real achievements, these significant steps may account for why his name came to be attached to Gregorian chant.