"Sumer is icumen in" is the incipit of a medieval English round or rota of the mid-13th century; it is also known variously as the Summer Canon and the Cuckoo Song.

The line translates approximately to "Summer has come" or "Summer has arrived". The song is written in the Wessex dialect of Middle English. Although the composer's identity is unknown today, it may have been W. de Wycombe or a monk at Reading Abbey, John of Fornsete [Wikidata]. The manuscript in which it is preserved was copied between 1261 and 1264.

This rota is the oldest known musical composition featuring six-part polyphony.

It is sometimes called the Reading Rota because the earliest known copy of the composition, a manuscript written in mensural notation, was found at Reading Abbey; it was probably not drafted there, however. The British Library now retains this manuscript. A copy of the manuscript in stone relief is displayed on the wall of the ruined chapter house of Reading Abbey.

A rota (Latin for 'wheel') is a type of round, which in turn is a kind of part song. To perform the round, one singer begins the song, and a second starts singing the beginning again just as the first gets to the point marked with the red cross in the first figure below. The length between the start and the cross corresponds to the modern notion of a bar, and the main verse comprises six phrases spread over twelve such bars. In addition, there are two lines marked "Pes", two bars each, that are meant to be sung together repeatedly underneath the main verse. These instructions are included (in Latin) in the manuscript itself:

Hanc rotam cantare possunt quatuor socii. A paucioribus autem quam a tribus uel saltem duobus non debet dici preter eos qui dicunt pedem. Canitur autem sic. Tacentibus ceteris unus inchoat cum hiis qui tenent pedem. Et cum uenerit ad primam notam post crucem, inchoat alius, et sic de ceteris. Singuli de uero repausent ad pausaciones scriptas et non alibi, spacio unius longe note.

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"Sumer is icumen in" in modern notation: