Plainsong or plainchant (calque from the French plain-chant; Latin: cantus planus) is a body of chants used in the liturgies of the Western Church. When referring to the term plainsong, it is those sacred pieces that are composed in Latin text. Plainsong was the exclusive form of the Western Christian church music until the ninth century, and the introduction of polyphony.

The monophonic chants of plainsong have a non-metric rhythm, which is generally considered freer than the metered rhythms of later Western music. They are also traditionally sung without musical accompaniment, though recent scholarship has unearthed a widespread custom of accompanied chant that transcended religious and geographical borders.

There are three types of chant melodies that plainsongs fall into: syllabic, neumatic, and melismatic. The free flowing melismatic melody form of plainsong is still heard in Middle Eastern music being performed today.

Although the Catholic Church and the Eastern Orthodox churches did not split until long after the origin of plainsong, Byzantine chants are generally not classified as plainsong.

Plainsong developed during the earliest centuries of Christianity, influenced possibly by the music of the Jewish synagogue and certainly by the Greek modal system. It has its own system of notation.

As the number of chants in the church's repertoire increased, officials needed a better way to standardize the music. A unique form of musical notation was developed to help standardize the music and provide a reference for the performers and audience alike. The musical notations that were used were called neumes, and they are employed on a four-line staff, unlike the five-line staff we are accustomed to today. The earliest neumes were marks placed above the chant's words to help the performer remember the piece's melody. They showed the general shape of the melody, but did not specify the pitches or intervals that needed to be sung, so the melody had to be learned by ear. It was not until the eleventh century that a notation system was perfected that placed the neumes on a four-line staff. This allowed the music to be written down accurately. In the example from the plainsong mass Orbis Factor, the notes for each syllable of the text are grouped together. Their position on the lines shows their pitch relative to one another, and the dots after some notes indicate a lengthening of the note. The vertical stroke after the word Kyrie indicates where the singer may take a breath.

Most of the early plainsong manuscripts have been destroyed due to war, purposeful destruction and natural causes such as water, fire, and poor environmental conditions. The Toledo Cathedral in Spain has one of the world's largest collections of indigenous plainsong manuscripts devoted to Western Christianity. Their collection consists of 170 volumes of plainsong chants for the procession, Mass, and Office.

There are three methods of singing psalms or other chants, responsorial, antiphonal, and solo. In responsorial singing, the soloist (or choir) sings a series of verses, each one followed by a response from the choir (or congregation). In antiphonal singing, the verses are sung alternately by soloist and choir, or by choir and congregation. It is probable that even in the early period the two methods caused the differentiation in the style of musical composition which is observed throughout the later history of plain chant, the choral compositions being of a simple kind, the solo compositions more elaborate, using a more extended compass of melodies and longer groups of notes on single syllables. The last type of plainsong performance is the solo performed by the choir or the individual performer. A marked feature in plainchant is the use of the same melody for various texts. This is quite typical for the ordinary psalmody in which the same formula, the "psalm tone", is used for all the verses of a psalm, just as in a hymn or a folk song the same melody is used for the various stanzas.