Ilha Grande literal translation: Big Island is an island located off the coast of Rio de Janeiro state, Brazil, and part of the municipality of Angra dos Reis. The island is largely undeveloped and noted for its scenic beauty, which includes tropical beaches, luxuriant vegetation and a rugged landscape. The land area is 193 km² and the highest point is Pico da Pedra D'Água, at 1031 m. It once held a notorious prison, now closed.

Ilha Grande is one of the most pristine remnants of Brazil's Atlantic rainforest, one of the richest ecosystems in the world, and a hotspot for biodiversity and conservation. It holds some of the largest remaining populations of many endangered species, including the red-ruffed fruitcrow (Pyroderus scutatus), the brown howler monkey (Alouatta fusca), the maned sloth (Bradypus torquatus) the red-browed Amazon parrot (Amazona rhodocorytha), and the broad-snouted caiman (Caiman latirostris). The seas around the island, which are also protected, feature a unique convergence of tropical, subtropical, and temperate-zone marine life, and may be the only waters in the world where it is possible to see corals and tropical fish along with Magellanic penguins and Southern right whales.

The entire island is a protected area, with most of its territory included in Ilha Grande State Park, and the rest subject to stringent development restrictions. Small-scale ecotourism, however, is encouraged, and the island, which is roadless and off-limits to cars, features over 150 km of hiking trails connecting the handful of coastal villages and hamlets, where lodging is available, to one another and to the many beaches, mountain peaks, waterfalls, and pristine forests.

The island was the scene of a devastating mudslide on January 1, 2010 killing at least 19 people.