Prithu (Sanskrit: पृथु, Pṛthu, lit. "large, great, important, abundant") is a sovereign (chakravarti), featured in the Puranas. According to Hinduism, he is an avatar (incarnation) of the preserver god—Vishnu. He is also called Prithu, Prithi and Prithvi Vainya, literally, "Prithu — the son of Vena".

Prithu is "celebrated as the first consecrated king, from whom the earth received her (Sanskrit) name, Prithvi." He is mainly associated with the legend of his chasing the earth goddess, Prithvi, who fled in the form of a cow and eventually agreed to yield her milk as the world's grain and vegetation. The epic Mahabharata, Vishnu Purana, and the Bhagavata Purana describe him as a part-avatar (incarnation) of Vishnu.

Prithu was born without female intervention(without a womb), thus being a ayonija ("born without (the participation) of the yoni"), Prithu is untouched by desire and ego and can thus control his senses to rule with dharma.

The legend of Prithu is found in the epic Mahabharata and most Puranas; the Vishnu Purana, Bhagavata Purana and the Padma Purana are the most detailed.

The Vishnu Purana, the Matsya Purana, the Brahma Purana, the Bhagavata Purana and the Harivamsha state that King Vena was the son of King Anga, from the lineage of the pious Dhruva, though the scriptures differ in the number and names of the generations between Dhruva and Anga. The Padma Purana however states that Anga descends from the sage Atri.

The Vishnu Purana states that Vena's mother was Sunitha, the daughter of Mrityu. Vena was an evil king, who neglected Vedic rituals. Thus the rishis (sages) killed him, leaving the kingdom without an heir and in famine due to the anarchy of Vena. So to create a son, the sages rubbed Vena's thigh, out of which first appeared a dark dwarf hunter, a symbol of Vena's evil. The dwarf was known as Nishada, the originator of his namesake race. Since the sins of Vena had gone away as the dwarf, the body was now pure. On further churning, Prithu emerged from right arm of the corpse. The Bhagavata Purana adds that when the sage churned the arms of Prithu, a couple emerges: Prithu and Archi, who will be his wife.

The Vishnu Purana continues: Prithu had the sign of Vishnu's chakra (discus) on his right hand. The creator-god Brahma recognized Prithu as an avatar of Vishnu, owing to this. His power is said to superior to the gods. The text states the chakra is the sign of an universal emperor (chakravarti, lit. "in whom the chakra abides"). However according to Oldham, the title chakravarti may be derived from the birthmark, and may not be indicative of universal dominion. In the Bhagavata Purana specifics that the chakra mark is on Prithu's right palm and a lotus mark on the soles of his feet.

In the Vishnu Purana, a celestial bow called Ajagava and arrows etc. were gifted by the heavens to Prithu. The Bhagavata Purana describes several gifts like throne, ornaments, royal umbrella, crown, sceptre, sword, shield, bow and arrows, horses, cows and other royal signina as gifts from various deities.