Rennell Island, locally known as Mugaba, is the main island of two inhabited islands that make up the Rennell and Bellona Province in the nation state of Solomon Islands. Rennell Island has a land area of 660 square kilometres (250 sq mi) and is about 80 kilometres (50 mi) long and 14 kilometres (8.7 mi) wide. It is the second largest raised coral atoll in the world with the largest lake in the insular Pacific, Lake Tegano, a lake that is listed as a World Heritage Site. Rennell Island has a population of about 1,840 persons of Polynesian descent who primarily speak Rennellese, Pijin and some English. Rennell and Bellona Islands are two of the few islands in the otherwise Melanesian Solomon Islands archipelago classified as a Polynesian outlier.

The island lies 236 kilometres (127 nmi; 147 mi) south of Honiara and 1,900 kilometres (1,000 nmi; 1,200 mi) north-east of Brisbane. The provincial capital, Tigoa, is at the western end of the island.

Bauxite mining and logging on the west side of the island was poorly regulated, resulted in spills, and seriously damaged the ecology and economy of the island between 2011 and 2021.

Between 2000 and 1600 BC, people belonging to the Lapita Culture first occupied the island. The next settlement occurred on both Rennell and Bellona around 130 BC, with another major occupation in about 1000 AD. Later settlement of Rennell occurred prior to 1400 AD by Polynesians from Uvea, now Wallis Island.

Captain Butler of HMS Walpole was the first European to discover the islands in 1801. In 1933 the Templeton Crocker Expedition discovered several endemic species on the islands. During the Pacific campaign of World War II Japanese Rufe floatplanes operated from Lake Tegano until American Catalinas used the lake as a base after 1943. The Battle of Rennell Island was the last major naval battle of the Guadalcanal Campaign which occurred between 29 January 1943 and 30 January 1943. At the end of the war, eight Catalinas were scuttled in the lake. The island was later visited by a series of Danish scientific expeditions.

The present-day inhabitants say their ancestors, Kaitu'u, arrived around 1400AD from Ubea, now Wallis and Futuna Islands, crossed the Pacific Ocean and settled on Rennell and Bellona Islands, in the Solomon Islands. One of the villages in Lake Tegano is called Hutuna which is the Rennellese interpretation of Wallis and Futuna.

The islands are of volcanic origin with basement rocks forming between the late Cretaceous and early Eocene. Tectonic movements raised the seabed to allow coral building. The whole Rennell area is thought to have been initially deposited as coralline algal limestone and then dolomitized. This dolomitic reef complex is overlain by younger undolotomized reef limestone. The island is the second largest upraised coral atoll in the world. It is largely unmodified and includes the largest lake in the South Pacific.

The island is about one hour's flight in a twin engine prop aircraft south of Honiara. It is almost totally surrounded by 120 to 150 metres (390 to 490 feet) cliffs, with the eastern end dominated by a large lake, while the western end is relatively flat with rolling forested hills. One road, known as the Copperhead Road, traverses the length of the island from the Tigoa airstrip in the west to the Labagu port area in the south, with a branch heading east towards the lake.