Grotta di San Giovanni, Natural cave in Domusnovas, Italy.

Grotta di San Giovanni is a karst cave in Domusnovas, Sardinia, formed by an underground river that caused a limestone mass to collapse. Its main passage runs 850 metres between a south entrance at the foot of Monte Acqua and a monumental north entrance in the Oridda valley, with two further levels, the fossil Ramo Bobore and the active Su Stampu de Pireddu, bringing the total to about 2000 metres.

Pottery shards found inside show the cave was used as a shelter in prehistoric times, and the remains of thick walls that once fortified its north and south entrances survive. These walls stood until the 19th century, when Count Beltrami promoted a carriage road to carry material from the nearby Sa Duchessa mine; building it meant demolishing the chapel of San Giovanni at the north entrance, later rebuilt in the valley beside the south entrance.

The cave's name comes from a chapel of San Giovanni that stood inside it near the north entrance until the 19th century, when Count Beltrami had it demolished to build the carriage road. It was rebuilt in the valley beside the south entrance, where it still stands among olive trees. The north and south mouths were once fortified with prehistoric walls, which survived until that same century.

The cave is crossed on foot along the former road, now closed to traffic, with renewed lighting along the 850 m main passage. The south entrance has a car park and a rest area; the north entrance is reached via the Marganai road, where a barrier opens from 9 to 16. An interactive multilingual audio guide is available, and the route is accessible to visitors with disabilities.

Since June 2020 the cave has been the first in the world with Wi-Fi coverage along its entire 850-metre length. It is also the longest of seven natural caves worldwide that are crossed by a road, ahead of Mas d'Azil in France (420 m) and Fengyang Pass in China (370 m).