A reservoir (French: réservoir) is an artificial lake created by flooding land behind a dam. Some of the world's largest lakes are reservoirs.

Surveyors have to find river valleys which are deep and narrow; the valley sides can then act as natural walls. The best place for building a dam has to be determined. If necessary, humans have to be rehoused and/or historic sites have to be moved, e.g. the temples of Abu Simbel before the construction of the Aswan Dam, creating Lake Nasser.

A reservoir may also be a term used by engineers to describe an enclosed container for a fluid; often, but not always, as part of a system of piping in which the contents are under pressure. An alternative name for this latter variant is a pressure tank.

In thermodynamics a reservoir is something that can be the source or sink for the transfer of heat into or out of a system, without appreciably changing its own temperature.

A reservoir can also be any container for storing a reserve (generally of a liquid), such as the ink reservoir of a dip pen, or that which is stored, as in "retirees are an untapped reservoir of talent".

Yet another meaning of reservoir is an accumulation of oil or gas beneath the Earth's surface - see oil field.

Reservoir may also refer to a carrier of a virus or parasite for which they are not pathogenic.

See also: List of reservoirs and dams