In Hindu mythology, the avatar of Vishnu that appears as a fish to save Manu from a great flood is Matsya.
Matsya is the first avatar in the commonly cited Dashavatara, Vishnu’s ten principal incarnations. In the best-known version of the story, the righteous king Manu discovers a tiny fish and protects it. The fish grows dramatically, eventually revealing itself as Vishnu.
Matsya warns Manu of an impending flood and instructs him to build a boat. Manu preserves the seven sages, seeds of plants, and representatives of living beings. In some tellings, Matsya also defeats the demon Hayagriva and recovers the Vedas after they were stolen.
Matsya is often confused with Kurma, Vishnu’s tortoise avatar, and Varaha, the boar avatar. The flood narrative also resembles other ancient flood traditions, but its specifically Hindu form centers on Manu and Vishnu’s fish incarnation.