Neptune's Grotto (Italian: Grotta di Nettuno; Catalan: Cova de Neptú) is a stalactite cave near the town of Alghero on the island of Sardinia, Italy.

The cave was discovered by local fishermen in the 18th century and has since developed into a popular tourist attraction.

The combined length of the cave system is estimated to be around 4 km (2.5 mi), but only approximately 300 m (1,000 ft) are accessible to the public.

[2] Inside are passages of lit stalactite and stalagmite formations, and a 120-metre (390 ft) long saltwater lake, which is at sea level.

The grotto is widely visited, and during the peak tourist season in August, can contain around 200 people at a time.