Cas Cay is a fifty-acre (200,000-square-metre) undeveloped island, situated a mile south of Red Hook on the eastern side of Saint Thomas in the United States Virgin Islands.

[3] Cas Cay is a tropical island, which has a hundred feet tall hill with bird species such as the Red-billed- and White-tailed tropicbird.

It is also home to endemic plant species such as orchids, Tabebuia, gumbo-limbos, as well as the seagrape, manchineel and numerous mangroves.

[6] Besides the mangrove ecosystem, the wildlife sanctuary, the natural whirlpool and blowhole, many visit the island for recreational activities such as snorkeling, scuba diving, kayaking and bird watching.

A 225 ha area, encompassing Cas Cay, neighbouring Bovoni and Patricia Cays, along with the Mangrove Lagoon marine reserve, has been recognised as an Important Bird Area (IBA) by BirdLife International because it supports populations of green-throated caribs, Antillean crested hummingbirds, pearly-eyed thrashers and Lesser Antillean bullfinches.