Hapi was the god of the annual flooding of the Nile in ancient Egyptian religion. The flood deposited rich silt (fertile soil) on the river's banks, allowing the Egyptians to grow crops. Hapi was greatly celebrated among the Egyptians. Some of the titles of Hapi were "Lord of the Fish and Birds of the Marshes" and "Lord of the River Bringing Vegetation". Hapi is typically depicted as an androgynous figure with a big belly and large drooping breasts, wearing a loincloth and ceremonial false beard.

The annual flooding of the Nile occasionally was said to be the Arrival of Hapi. Since this flooding provided fertile soil in an area that was otherwise desert, Hapi symbolised fertility. He had large female breasts because he was said to bring a rich and nourishing harvest. Due to his fertile nature he was sometimes considered the "father of the gods", and was considered to be a caring father who helped to maintain the balance of the cosmos, the world or universe regarded as an orderly, harmonious system. He was thought to live within a cavern at the supposed source of the Nile near Aswan. The cult of Hapi was mainly located at the First Cataract named Elephantine. His priests were involved in rituals to ensure the steady levels of flow required from the annual flood. At Elephantine the official nilometer, a measuring device, was carefully monitored to predict the level of the flood, and his priests must have been intimately concerned with its monitoring.

Hapi was not regarded as the god of the Nile itself but of the inundation event. He was also considered a "friend of Geb", the Egyptian god of the earth, and the "lord of Neper", the god of grain.