Samhain (pronounced /ˈsɑːwɪn/, /ˈsaʊ.ɪn/, or /ˈsaʊn/ in English; from Irish samhain [ˈsˠaunʲ], cf. Scottish Gaelic samhainn [ˈsavɯɲ], Old Irish samain [ˈsaṽɨnʲ] "summer's end", from sam "summer" and fuin "end") is a festival held at the end of the harvest season in Gaelic and Brythonic cultures. Principally a harvest festival, it also has aspects of a festival of the dead. It had its roots in ancient Celtic polytheism, and continued to be celebrated through medieval times, and is seen as contributing to the modern celebration of Halloween. Many scholars believe that it was the beginning of the Celtic year.

The term "Samhain" derives from the name of a month in the ancient Celtic calendar, in particular the first three nights of this month, with the festival marking the end of the summer season and the end of the harvest. Samhain was also called the Féile Moingfhinne ie "Festival of Mongfind". According to Cormac's Glossary, Mongfind (mod.Irish spelling Mongfhionn) was a goddess the pagan Irish worshipped on Samhain. The Gaelic festival became associated with the Catholic All Souls' Day, and appears to have influenced the secular customs now connected with Halloween. Samhain is also the name of a modern festival in various currents of Neopaganism that are based on, or inspired by, Gaelic traditions.

Samhain and an t-Samhain are also the Irish and Scottish Gaelic names of November, respectively.