Unowned property includes tangible, physical things that are capable of being reduced to being property owned by a person but are not owned by anyone. Bona vacantia (Latin for "ownerless goods") is a legal concept associated with the unowned property, which exists in various jurisdictions, with a consequently varying application, but with origins mostly in English law.

Nearly every piece of land on the Earth is a property and has a maintainer (owner). The class of objects, "unowned things", are objects which are not yet property; either because it has been agreed by sovereign nations that no one can own them, or because no person, or other entity, has made a claim of ownership. The most common unowned things are asteroids. The UN's Outer Space Treaty does not address the issue of private ownership of natural objects in space. All asteroids remain unowned things until some person or entity makes a claim of property right to one of them.

In an experimental legal case of first impression, a lawsuit for a declaratory judgment was filed in a United States Federal Court to determine the lawful owner of Asteroid 433 Eros. 433 Eros was claimed as property by Gregory W. Nemitz of Orbital Development. According to the homestead principle, Nemitz argued that he had the right to claim ownership of any celestial body that he made use of; he claimed he had designated Eros a spacecraft parking facility and wished to charge NASA a parking and storage fee of twenty cents per year for its NEAR Shoemaker spacecraft that is permanently stored there. Nemitz's case was dismissed due to lack of standing and an appeal denied.