An island can be owned by a private person, a company, or a government, and the rules that govern that ownership come from wherever the island physically sits. So the question of who owns an island has two layers: who holds title to the land above the waterline, and who controls everything around it. In almost every case, those are different parties. The land can be private property; the water, the seabed, and often the beach belong to the state or the public.
Who Can Hold Title to an Island
Private individuals and corporations buy and sell islands the same way they buy and sell any other real estate. A listing, a price, a title search, a closing. The due diligence is heavier because many islands have passed through informal transfers, colonial-era grants, or unresolved indigenous claims, and some sit in jurisdictions where the land registry is a single courthouse ledger. Legal access is its own problem. An island you cannot lawfully reach by boat or aircraft is close to worthless, and docking or landing rights across neighboring mainland property may need to be negotiated as a separate deal.
Title insurance helps less than buyers expect. Standard policies routinely exclude survey disputes, boundary conflicts, mineral rights, and adverse possession claims, which are precisely the issues most likely to come up on a parcel whose edges meet tidal water.
Foreign Ownership Rules
Many countries limit who can hold coastal or island land. Mexico’s constitution bars direct foreign ownership of land within 50 kilometers of the coast, but foreign buyers can hold property through a bank trust in which a Mexican bank acts as trustee. Chile reserves public coastal lands for Chilean nationals, with narrow exceptions for foreign residents who get naval approval. Finland’s autonomous Åland Islands require government approval before anyone without local residency rights can buy real property.1Library of Congress. Restrictions on Land Ownership by Foreigners in Selected Jurisdictions For a foreign buyer, the holding structure often matters as much as the purchase price.
What Ownership Actually Covers
Owning the dry land does not mean owning the water or the ground beneath it. In the United States, the Submerged Lands Act confirms that each state holds title to the lands beneath navigable waters within its boundaries, including tidal lands up to the mean high-tide line and extending seaward for three geographical miles.2Office of the Law Revision Counsel. 43 US Code 1311 – Rights of States Your property line typically stops at the waterline.
The public trust doctrine adds another limit. Rooted in English common law and recognized across most U.S. states, it treats navigable waters and the lands beneath them as held in trust for public use. Even on a private island, the public generally keeps the right to navigate the surrounding waters by boat. Whether people can walk the beach or wade along the shoreline varies by state, and the case law is unsettled in many places. Fencing off the waterfront is not a safe assumption.
Islands Owned by Governments
Governments at every level hold islands. The federal government owns islands for military installations, national parks, and wildlife refuges. States hold islands for conservation and resource management. Local governments may hold smaller islands as open space or community recreation. Public access tracks the designated purpose. A national park is open; a military installation is not.
When a government decides it no longer needs an island, it can sell or transfer the property through a surplus process. That is how some private island sales originate. Former government land often carries deed restrictions on future use, environmental cleanup obligations, or retained easements for navigation aids such as lighthouses.
What You Can Build on a Private Island
Owning an island does not come with free rein to develop it. Federal environmental laws apply to island construction the same way they apply on the mainland, and the permitting burden is often heavier because the work sits at the intersection of land and water.
Any structure built in or over navigable waters requires a permit under the Rivers and Harbors Act of 1899. That covers docks, piers, seawalls, and breakwaters, and the U.S. Army Corps of Engineers administers the program. The prohibition reaches not just permanent structures but any work that affects the course, condition, or capacity of navigable waters, and the Corps’ authority extends to artificial islands and installations out to the edge of the outer continental shelf.3eCFR. 33 CFR 320.2 – Authorities to Issue Permits
Filling wetlands, dredging, or depositing material into waters of the United States also requires authorization under Section 404 of the Clean Water Act. The covered activities include foundations, seawalls, breakwaters, artificial islands, causeways, and temporary structures such as cofferdams and access roadways.4US Army Corps of Engineers. Section 404 of the Clean Water Act Low-impact work can sometimes proceed under a Nationwide Permit; larger projects need individual permits with full environmental review.5U.S. Army Corps of Engineers. Nationwide Permits
Islands frequently host endangered species. The Endangered Species Act makes it illegal to “take” a listed species, and courts read “take” broadly to include habitat destruction that harms or harasses listed wildlife.6U.S. Fish and Wildlife Service. Section 9 Prohibited Acts Sea turtle nesting beaches or migratory bird habitat on your island will shape what development is allowed. On top of all this, the Coastal Zone Management Act lets states run their own coastal permitting programs covering wetlands, hazards, public access, and the cumulative impact of development.7National Oceanic and Atmospheric Administration. Coastal Zone Management Act Section 309 Program Guidance 2026-2030 State review stacks on top of federal review, and approvals can take years.
Taxes and Reporting
Private islands generate property tax wherever they sit, assessed by the local taxing authority at the prevailing rate. Comparable sales data is thin, which sometimes leads to disputes over assessed value.
U.S. citizens who own islands abroad face additional reporting rules, though direct ownership of foreign real estate is not itself a specified foreign financial asset under FATCA and does not by itself trigger Form 8938.8Internal Revenue Service. Basic Questions and Answers on Form 8938 If the island is held through a foreign corporation, partnership, or trust, the interest in that entity is reportable once it crosses the filing thresholds, which vary by filing status and whether you live in the U.S. or abroad.9Internal Revenue Service. Do I Need to File Form 8938, Statement of Specified Foreign Financial Assets A foreign bank account used to manage the property triggers a separate FinCEN filing if it exceeds $10,000 in aggregate at any point during the year.10FinCEN.gov. Report Foreign Bank and Financial Accounts Penalties for missing these filings are steep, and they apply even when no additional tax is owed.
When Nations Claim Islands
For countries, the value of an island is usually not the land but the ocean around it. Under the United Nations Convention on the Law of the Sea, an island is a naturally formed area of land, surrounded by water, that stays above the surface at high tide. Each qualifying island generates a territorial sea extending up to 12 nautical miles from the baseline, giving the coastal state full sovereignty over the water column, seabed, and airspace, plus a contiguous zone out to 24 nautical miles for enforcement of customs, immigration, and sanitation laws.11United Nations. United Nations Convention on the Law of the Sea – Part II An exclusive economic zone can reach 200 nautical miles, giving the state exclusive rights to natural resources in the water and on the seabed.12United Nations. United Nations Convention on the Law of the Sea – Part V A single small island in the right place can generate hundreds of thousands of square miles of resource-rich ocean.
UNCLOS carves out an important exception. A rock that cannot sustain human habitation or independent economic activity gets a territorial sea but no exclusive economic zone and no continental shelf rights. Artificial islands built on reefs or sandbars do not qualify as islands at all and generate no maritime zones.13United Nations. United Nations Convention on the Law of the Sea – Part VIII That line explains why some states have poured concrete onto reefs and built airstrips on atolls: the aim is to convert what UNCLOS would treat as a rock into something closer to a habitable island.
Truly unclaimed islands essentially no longer exist. Nearly every piece of land has been surveyed and claimed by a sovereign state, and when new islands form through volcanic activity the nearest coastal state usually claims them quickly and without challenge. Disputed islands are harder. The arguments tend to turn on historical control, colonial-era boundaries, and which state has actually administered the territory. In the 1992 El Salvador/Honduras frontier dispute, the International Court of Justice found that long, undisputed control by one party creates a strong presumption of sovereignty, and that belated protests carry little weight.14International Court of Justice. Land, Island and Maritime Frontier Dispute (El Salvador/Honduras) These disputes are resolved through diplomatic negotiation, mediation, or submission to an international court or arbitral tribunal.