Veni, Veni, Emmanuel (O Come, O Come, Emmanuel) is one of the oldest Advent hymns still in regular use among most Christian denominations. The original text is based on the ninth-century "O" Antiphons. These were seven antiphons sung one per night in the week preceding Christmas. The name refers to the opening words of each antiphon: "O Sapienta" (Wisdom); "O Adonai" (Lord of Might); "O Radix Jesse" (Root of Jesse); "O Clavis David" (Key of David); "O Oriens" (Light of the East); "O Rex gentium" (King of Nations); "O Emmanuel" (Deliverer). The initials form an acrostic – S-A-R-C-O-R-E – which, read backward, is "Ero cras" (I will be there tomorrow), referring to Jesus' arrival on Christmas Day in response to the petitions of the verses.

By the thirteenth century, five of the antiphons had been paraphrased into the Latin hymn "Veni, Emmanuel." A refrain was added as well, "Gaude, Gaude, Emmanuel; Nascetur pro te, Israel" (Rejoice, Rejoice, Emmanuel; Shall come to thee, O Israel), the repetition of text being somewhat unusual for Gregorian chant. The music is a plainsong, probably dating from the same time, documented in a two-voice form as part of a fifteenth-century processional found in a French nunnery. The tune is a modal chant, often performed in its two-voice version. Like most early antiphons, the melody is primarily syllabic, with a very even rhythm, and moving within a limited range, the largest interval being a fourth.

The hymn gained widespread use with a four-part harmonization and English translation, published in the 1850s by Thomas Helmore and John M. Neal. Neal was a translator of several hymns, while Helmore revived the use of Gregorian chant in Anglican services. In modern performances, the addition of an organ accompaniment serves to magnify the words of the refrain.