A domestic partnership is an intimate relationship between people, usually couples, who live together and share a common domestic life but who are not married (to each other or to anyone else). People in domestic partnerships receive legal benefits that guarantee right of survivorship, hospital visitation, and other rights.

The term is not used consistently, which results in some inter-jurisdictional confusion. Some jurisdictions, such as Australia, New Zealand, and the U.S. states of California, Maine, Nevada, Oregon and Washington use the term "domestic partnership" to mean what other jurisdictions call civil union, civil partnership, or registered partnership. Other jurisdictions use the term as it was originally coined, to mean an interpersonal status created by local municipal and county governments, which provides an extremely limited range of rights and responsibilities.

Some legislatures have voluntarily established domestic partnership relations by statute instead of being ordered to do so by a court. Although some jurisdictions have instituted domestic partnerships as a way to recognize same-sex marriage, statutes do exist which provide for recognition of opposite-sex domestic partnerships in many jurisdictions.

In some legal jurisdictions, domestic partners who live together for an extended period of time but are not legally entitled to common-law marriage may be entitled to legal protection in the form of a domestic partnership. Some domestic partners may enter into nonmarital relationship contracts in order to agree, either verbally or in writing, to issues involving property ownership, support obligations, and similar issues common to marriage. (See effects of marriage and palimony.) Beyond agreements, registration of relationships in domestic partnership registries allow for the jurisdiction to formally acknowledge domestic partnerships as valid relationships with limited rights.

Although the terms are sometimes used interchangeably, a Domestic Partnership, Same Sex Marriage or Civil Union are each separate and distinct legal concepts. The domestic partnership is a legal relationship between two people of the same or opposite sex who live together and share a domestic life, but are not married or joined by a civil union nor are blood relatives. It may be established by contract between the parties, but more often by registration according to procedures established by a state or municipal government. Benefits granted under a domestic partnership vary among different jurisdictions. Some accord full health benefits, others only a right of visitation. In still other jurisdictions, registered domestic partners are accorded a legal status similar to that of a married person with respect to matters of probate, guardianships, conservatorships, inheritance, protection from abuse, and related matters.

Since the 2015 US Supreme Court's decision legalizing same-sex marriage, there have been fewer US domestic partnerships registered, but in many jurisdictions they are still allowed for couples of the same gender or different genders who do not want to marry but still would like to be eligible for certain benefits. Many couples opt for a domestic partnership after comparing the potential tax consequences of being married.

In August 1979, gay rights activist Tom Brougham proposed a new category of relationship called "domestic partnership". Initially, the requirements were that only two people who resided together and were qualified to marry except that they were the same gender. Additional requirements were later added for the partners to maintain mutual financial responsibility and for both to be at least eighteen years old and able to enter into a legal contract.

In 1982, Brougham's definition was modified by Supervisor Harry Britt, a gay man appointed to replace Harvey Milk. Britt's version was adopted and passed by the San Francisco Board of Supervisors, but Dianne Feinstein, mayor of San Francisco at the time, came under intense pressure from the Catholic Church and vetoed the bill. In 1989, a domestic partnership law was adopted in San Francisco. However, voters repealed the domestic partnership law by initiative; a modified version was reinstated by another voter initiative, 1990's Proposition K, also written by Britt. Currently, the city still offers a domestic partnership status separate and differing in benefits from that offered by the state; city residents can apply for both.