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Ballast Island, also known as Barasu Island (Japanese: バラス島, romanized: Barasu-shima), is a small uninhabited coral island or cay in the Yaeyama chain off the coast of mainland Japan, located between the islands of Iriomote and Hatoma. The island sits at the center of a coral reef composed primarily of species of the stony coral genus Acropora, most numerously Acropora muricata. At least four goby species have been found at the island, as well as a specimen of the moray eel Gymnothorax ryukyuensis.
Two small islets on the site had formed by the 1960s. A mass bleaching of the reef's corals from July to September 1998 led to the merging of the two islets, accelerated by the impact of several typhoons in 1998 and 1999. Ballast Island reached a maximum land area of around 10,000–20,000 square metres (2.5–4.9 acres) in late 1999, before steadily declining over the following years. As of 2016[update], the island had a surface area of around 2,000–2,400 square metres (0.5–0.6 acres), a size it has maintained since 2009. While uninhabited, the island is visited on snorkeling tours departing from Iriomote, allowing access to the surrounding coral reefs.