Jack in the Green, also known as Jack o' the Green, is an English folk custom associated with the celebration of May Day. It involves a pyramidal or conical wicker or wooden framework that is decorated with foliage being worn by a person as part of a procession, often accompanied by musicians.

May Day is a European festival of ancient origins marking the beginning of summer, usually celebrated on 1 May, around halfway between the Northern Hemisphere's Spring equinox and June solstice. Festivities may also be held the night before, known as May Eve. Traditions often include gathering wildflowers and green branches, weaving floral garlands, crowning a May Queen, and setting up a Maypole, May Tree or May Bush, around which people dance and sing. Bonfires are also a major part of the festival in some regions. Regional varieties and related traditions include Walpurgis Night in central and northern Europe, the Gaelic festival Beltane, the Welsh festival Calan Mai, and May devotions to the Blessed Virgin Mary. It has also been associated with the ancient Roman festival Floralia.