Jarilo, Yarilo, Jarovit or Gerovit, was a major male Proto-Slavic deity of vegetation, fertility and spring, also associated with war and harvest. The Slavic root 'jar' or 'yar' means spring or summer.

He was a fairly typical life-death-rebirth deity, believed to be (re)born and killed every year. His mythical life cycle followed the yearly life of various wheat plants, from seeding through vegetation to harvest.

Jarilo was a son of the supreme Slavic god of thunder, Perun, his lost, missing, tenth son, born on the last night of February, the festival of Velja Noć (Great Night), the pagan Slavic celebration of the New Year. On the same night, however, Jarilo was stolen from his father and taken to the world of the dead, where he was adopted and raised by Veles, Perun's enemy, Slavic god of the underworld and cattle. The Slavs believed the underworld to be an ever-green world of eternal spring and wet, grassy plains, where Jarilo grew up guarding the cattle of his stepfather. In the mythical geography of ancient Slavs, the land of the dead was assumed to lie across the sea, where migrating birds would fly every winter. With the advent of spring, Jarilo returned from the underworld, that is, bringing spring and fertility to the land. Spring festivals of Jurjevo/Jarilo that survived in later folklore celebrated his return.

The first of the gods to notice Jarilo's return to the living world was Morana, a goddess of death and nature, and also a daughter of Perun and Jarilo's twin-sister. The two of them would fall in love and court each other through a series of traditional, established rituals, imitated in various Slavic courting or wedding customs. The divine wedding between brother and sister, two children of the supreme god, was celebrated in a festival of summer solstice, today variously known as Ivanje or Ivan Kupala in the various Slavic countries. This sacred union of Jarilo and Morana, deities of vegetation and of nature, assured abundance, fertility and blessing to the earth, and also brought temporary peace between two major Slavic gods, Perun and Veles, signifying heaven and underworld. Thus, all mythical prerequisites were met for a bountiful and blessed harvest that would come in late summer.