Matsya is the Vishnu avatar who takes the form of a fish to save Manu from a catastrophic flood.
In the best-known versions, Manu first encounters Matsya as a tiny fish and protects it. The fish grows until Manu releases it into the sea, then warns him that a great deluge is coming. Manu builds a boat and gathers what must survive; in later tellings, this includes the seven sages, seeds, and living creatures.
Matsya guides the boat through the flood, commonly by allowing Manu to fasten it to the fish’s horn. The vessel reaches safety near the northern mountains, often identified with the Himalayas. Manu then becomes the ancestor of a renewed human population.
The story has a long textual history. The earliest extant account appears in the Shatapatha Brahmana, where the fish is not yet specifically Vishnu. Later epic and Puranic traditions identify the savior fish with Vishnu and add details such as the recovery of the Vedas from the demon Hayagriva.