A roommate is a person with whom one shares a living facility such as a room or dormitory except when being family or romantically involved. Similar terms include dormmate, suitemate, housemate, or flatmate ("flat": the usual term in British English for an apartment). Flatmate is the term most commonly used in New Zealand, when referring to the rental of an unshared room within any type of dwelling. Another similar term is sharemate (shared living spaces are often called sharehouses in Australia and other Commonwealth countries). A sharehome is a model of household in which a group of usually unrelated people reside together. The term generally applies to people living together in rental properties rather than in properties in which any resident is an owner occupier. In the UK, the term "roommate" means a person living in the same bedroom, whereas in the United States and Canada, "roommate" and "housemate" are used interchangeably regardless whether a bedroom is shared, although it is common in US universities that having a roommate implies sharing a room together. This article uses the term "roommate" in the US sense of a person one shares a residence with who is not a relative or significant other. The informal term for roommate is roomie, which is commonly used by university students.

The most common reason for sharing housing is to reduce the cost of housing. In many rental markets, the monthly rent for a two- or three-bedroom apartment is proportionately less per bedroom than the rent for a one-bedroom apartment (in other words, a three-bedroom flat costs more than a one-bedroom, but not three times as much). By pooling their monthly housing money, a group of people can achieve a lower housing expense at the cost of less privacy. Other motivations are to gain better amenities than those available in single-person housing, to share the work of maintaining a household, and to have the companionship of other people.

People become roommates when they move into a rental property, with one or more of them having applied to rent the property through a real estate agent, being accepted and having signed a lease.