Beltane () is the Gaelic May Day festival. Commonly observed on the first of May, the festival falls midway between the spring equinox and summer solstice in the northern hemisphere. The festival name is synonymous with the month marking the start of summer in Ireland, May being Mí na Bealtaine. Historically, it was widely observed throughout Ireland, Scotland, and the Isle of Man. In Irish the name for the festival day is Lá Bealtaine ([l̪ˠaː ˈbʲal̪ˠt̪ˠənʲə]), in Scottish Gaelic Latha Bealltainn ([l̪ˠaː ˈpjaul̪ˠt̪ɪɲ]), and in Manx Gaelic Laa Boaltinn/Boaldyn. Beltane is one of the principal four Gaelic seasonal festivals—along with Samhain, Imbolc, and Lughnasadh—and is similar to the Welsh Calan Mai.

Bealtaine is mentioned in the earliest Irish literature and is associated with important events in Irish mythology. Also known as Cétshamhain ('first of summer'), it marked the beginning of summer and was when cattle were driven out to the summer pastures. Rituals were performed to protect cattle, people and crops, and to encourage growth. Special bonfires were kindled, whose flames, smoke and ashes were deemed to have protective powers. The people and their cattle would walk around or between bonfires, and sometimes leap over the flames or embers. All household fires would be doused and then re-lit from the Bealtaine bonfire. These gatherings would be accompanied by a feast, and some of the food and drink would be offered to the aos sí. Doors, windows, byres and livestock would be decorated with yellow May flowers, perhaps because they evoked fire. In parts of Ireland, people would make a May Bush: typically a thorn bush or branch decorated with flowers, ribbons, bright shells and rushlights. Holy wells were also visited, while Bealtaine dew was thought to bring beauty and maintain youthfulness. Many of these customs were part of May Day or Midsummer festivals in parts of Great Britain and Europe.

Public celebrations of Beltane fell out of popularity, though some customs continue to be revived as local cultural events. Since the late 20th century, Celtic neopagans and Wiccans have observed a festival based on Beltane as a religious holiday. Some neopagans in the Southern Hemisphere celebrate Beltane on or around 1 November.