Ilha Grande (Portuguese pronunciation: [ˈiʎɐ ˈɡɾɐ̃dʒi]), or "big island", is a 193 km² (75 sq mi) forested island located around 151 km (94 mi) off of the Atlantic coast of Angra dos Reis, Rio de Janeiro, Brazil, and about 450 km (280 mi) from São Paulo. A popular outdoor destination, the island remains largely undeveloped as much of the area falls within Ilha Grande State Park (Parque Estadual da Ilha Grande). Thus, the remainder of the island is subject to stringent development restrictions in order to preserve the natural environment; vehicles are not permitted and roads are virtually non-existent. For visitors, an hour-long, pre-paid boat excursion is the sole way to access the island. Several small villas and hamlets cater to researchers, tourists and nature-lovers, and around 2,000 people inhabit the largest town on the island, Vila do Abraão.

For almost a century, the Brazilian government declared the island off-limits, banning unauthorized entry, as it had contained an immigration quarantine station and, later, a maximum-security prison (Colônia Penal de Dois Rios, later known as Instituto Penal Cândido Mendes). The Cândido Mendes Penal Colony, which housed some of the most dangerous offenders in Brazil, was closed in 1994.

The highest point on Ilha Grande is the 1,031 m (3,383 ft) tall Pico da Pedra D'Água.

On 5 July 2019, Ilha Grande and Paraty were inscribed as a UNESCO World Heritage Site.