Nereo Cave is an underwater sea-cave situated on the north-west of Sardinia in the Coral riviera of Alghero, Italy. The name was given by the discovers in honour of the mythological figure Nereus, who is often billed as the Old Man of the Sea, father of the Nereids. The site is under the high limestone cliffs of Capo Caccia, 100 metres north of the famous Neptune's Grotto. The cave is considered the biggest marine cave in the Mediterranean Sea. With the many entrances, arches and tunnels, it is possible to make dives from 0 to 35 m, through long and large tunnels, air chambers and different ways. The walls are covered with red coral and yellow leptosamnia.

With the other cape of Punta Giglio and the Porto Conte Bay, the cave is part of a Marine reserve set up in 2003. Its flora and fauna are typically Mediterranean including groupers, lobsters, congers and moray eels, and thriving crustacean life.