A private island is a privately owned island. Privately owned islands exist all over the world, and many can be bought. There are many thousands of uninhabited islands in the world with potential for commercial development of tourist resorts or private recreational use. Commercial development of uninhabited islands can bring up ecological concerns, as many have a fragile environment. Some islands can be bought undeveloped, while other include roads and/or houses. Some celebrities like Diana Ross, Tony Curtis and Claudia Schiffer own their own private islands. Many cruise lines own private islands, such as Castaway Cay, which serve as exclusive ports of call for their ships. Others, like Mustique are owned by resort companies which fill them with hotels and leased villas.

There are widely varying politics regarding private islands; for instance, islands outside the coast of China cannot be purchased, but only leased from the government with a maximum leasing period of 50 years. Virtually all islands in the world are claimed by the various governments in the world, so the laws of the claiming country would apply and gaining sovereignty would be virtually impossible, but some people still try to set up their own micronations on islands. Since islands can, under international law, only be claimed if they are at least 30 cm above the high tide point, some have even attempted to build sovereign islands, like real-estate millionaire Michael Oliver's attempt at building a libertarian city-state called the Republic of Minerva in the southern Pacific Ocean.

"Private" islands in the United Kingdom, Brazil and other countries are not legally entirely private any foreshore, such as beaches, is owned by the state, and is hence publicly accessible property, despite what the owners of the land on the island may wish to claim.

According to a BBC's Fast Track report about private islands aired on June 27 2007, buyers must keep in mind:

• How far away is the island from the Mainland? A distance of more than 90 minutes is not recommended because of the need for access to hospitals and other emergency facilities.
• Is ownership clearly determined? Some countries impose restrictions on foreign ownership; for example, most Asian countries forbid foreigners from owning islands, with the exception of Malaysia. Other countries impose restrictions on development. Few islands are free held.
• Is drinking water available? Some islands require desalination plants, which can be very expensive.
• Is the host country politically stable? Islands in volatile countries are cheaper but the risks are higher.

List of island groups and celebrities who own(ed) them

• Marlon Brando - Tetiaroa, French Polynesia
• John Lennon - Dorinish Island in Clew Bay, County Mayo, Ireland
• Mel Gibson - Mago Island, Fiji
• Errol Flynn - Navy Island, Port Antonio, Jamaica
• Gene Hackman - Island in British Columbia, Canada
• Diana Ross - (Formerly) Taino, French Polynesia
• Ted Turner - St Phyllis Island, South Carolina, USA
• Malcolm Forbes - Laucala Island, Fiji
• Raymond Burr - Naitoumba Island, Fiji
• Sir Richard Branson Necker Is & Moskito Island - British Virgin Islands, Makepeace Is - Australia
• Aristotle Onassis - Skorpios Island, Greece
• David Copperfield - Musha Cay, Bahamas
• Charles A. Lindbergh - Illiec Island off the coast of Brittany, France
• Roca Family - Colom Island, Minorca, Spain
• Cinnamond Family - S'Espalmador Island, Formentera, Spain
• Bacardi Family - Island near Granada
• Disney family - Echo Island, San Juan Islands - Washington, USA
• Du Pont family - Cherry Island, Chesapeake Bay, Maryland, USA
• Lawrence Rockefeller - Sandy Cay, Jost Van Dyke BVI
• Baron Rothschild - Bell Island, Bahamas
• Nick Hexum - Melody Key, Florida
• Edward de Bono - Green Island - Australia, Tessera - Italy, Reklusia - Bahamas
• Princess Nina Aga Khan - Pellew Island, Port Antonio, Jamaica
• Dean Kamen - North Dumpling Island, Connecticut, USA
• Gianni Agnelli - Dino Island off the Calabrian coast
• Michael Ondaatje - Several islands, Mahone Bay, Ireland
• David H. Murdock - Lanai Island, Hawaii, USA