Hefker (Hebrew: הפקר) is ownerless property under rabbinic law. While some property is intrinsically ownerless, ordinary property can be made ownerless by the formal renunciation of its owner or by judicial action (as shown in Gittin 36b). Even absent such an action, property will become ownerless if the owner was a convert to Judaism who dies without Jewish heirs (as seen in Bava Batra 149a). Hefker re-emerged in the early 20th century as a symbol in European Hebrew and Yiddish literature.

One kind of property such as seas, rivers, and deserts, appears to be intrinsically ownerless and thus gets categorized as hefḳer. Similarly, hefker includes the trees of the forest, fish in the sea, and birds of the wilderness. This legal presupposition can be seen in the Jewish law code Shulchan Aruch (Ḥoshen Mishpaṭ, 273–274, with the gloss by Moses Isserles). This natural ownerless quality is reflected in a sugya (passage) from the Talmud, a foundational text for rabbinic law, that attributes ten ancient laws to Joshua upon entering Canaan (see Bava Kamma 81a).

The ownerless aspect of natural formations only applies to the property claims of Jews, or Christians, but not necessarily to kings and royalty, who exercise sovereign control of lakes, forests, and the like.

The renunciation of ownership in property, whether movable or immovable, in order to be valid must be made in the presence of three men (Nedarim 45a). The formula of such a renunciation is very simple: "This my property shall be hefḳer." If no one takes possession of the property during the first three days, the previous owner may retract his original statement, but not after that, although he can always acquire possession of it in the same manner as any one else (Nedarim 44a).

The renunciation is valid only when made in general terms, not when it is declared hefḳer only to a certain class and not to another class, as when one declares it hefḳer for the poor and not for the rich (Mishnah Peah 6:1, Bava Metsia 30b). It is disputed whether property can be made hefḳer if the owner abandons it to the world, except for one or two individuals.

Owners may renounce property for a period of time, such as the sabbath, during which others may take possession. There are rabbinic debates about whether owners can renounce property only for a limited set of potential recipients, such as designating an asset as abandoned for the poor and not for the wealthy.

In some cases, even absent a renunciation, property will become ownerless if it cannot be returned to the owner. Notably, property turns ownerless if the owner was a proselyte, a convert to Judaism, who dies without Jewish heirs (as seen in Bava Batra 149a). Likewise, property that is lost to sea, or similarly irretrievable, has the status of hefker. Talmudic literature mentions two other cases: chickens or certain other animals that escape the owner's possession and certain items that are forbidden to Jewish use (issur ha-na'ah), though this latter category is disputed among the rishonim rabbis.

Hefker, of various types, is acquired by the first person who cares to take possession of it. With a few exceptions, the manner of acquiring is the same in case of hefḳer as in other cases (see Alienation and Acquisition).