In Star Trek, a replicator is a machine that can create (and recycle) things. Replicators were originally seen to simply synthesize meals on demand, but in later series much larger non-food items appear. The technical aspects of replicated versus "real" things is sometimes a plot element.

Although previous sci-fi writers had speculated about the development of "replicating" or "duplicating" technology, the term "replicator" was not itself used until Star Trek: The Next Generation. In simple terms, it was described as a 24th century advancement from the 23rd century "food synthesizer" seen in Star Trek: The Original Series. In Star Trek: The Original Series, food was created in various colored cubes. In Star Trek: The Animated Series (1974), various types of realistic-looking food could be requested, as in the episode entitled "The Practical Joker". The mechanics of these devices were never clearly explained on that show. The subsequent prequel series, Star Trek: Enterprise, set in the 22nd century, featured a "protein resequencer" that could only replicate certain foods, so an actual chef served on board who used a hydroponic greenhouse where fruits and vegetables were grown. Additionally, that ship had a "bio-matter resequencer" which was used to recycle waste product into usable material.

Replicators reconstitute matter, converting pure energy into food, medicines and physical objects. A replicator can create any inanimate matter, as long as the desired molecular structure is on file, but it cannot create antimatter, dilithium, latinum, and (in the case of at least federation replicators) living things of any kind; for the last case, non-canon works such as the Star Trek: the Next Generation Technical Manual state that, though the replicators share the same technology with transporters, the resolution used is too low to create living tissue. However, other replicators, such as the ones used by the aliens in the TNG episode "Allegiance", could create living things, including the brain's many trillions of dendritic connections where memory is stored.

One of the most important pieces of technology in the Star Trek universe, the replicator, is used primarily to provide food and water on board starships, thus eliminating the need to stock most provisions (though starships, starbases, and other installations still stock some provisions for emergencies, such as in cases of replicator failure or an energy crisis). On Star Trek: Deep Space Nine, it was established that as long as there is an energy source to power life support, replication is used to provide breathable air on ships and starbases (and to disassemble the carbon dioxide exhaled by the crew), thus providing a seemingly endless supply of oxygen and eliminating the need to carry air tanks.

The technology is also used for producing spare parts, which makes it possible to repair most ship damage without having to return to a starbase. Other applications include replication of Starfleet uniforms and everyday objects such as toys and souvenirs. Replication is also used by the Holodeck program to allow food, clothes, and other objects belonging within a simulation to be used or consumed by the participants.

Starfleet's safety protocols prevent unauthorized replication of dangerous objects, such as weapons and poisonous substances.

Replicators can also convert matter into energy. Following that principle, the device can dismantle any object into subatomic particles. The ensuing energy can then be stored for future use or immediately applied in a subsequent replication. This process is referred to as "recycling", and is applied to everything from dirty dishes to outgrown children's clothes.

Replicator technology, even if produced on a larger scale, had not been able to be used to create complex objects such as shuttlecraft or starships (the production staff felt that being able to replicate entire starships "at the push of a button" would severely impact dramatic potential). However, in the Star Trek: Deep Space Nine episode "For the Cause", industrial replicators are used to replicate large components of ships, shuttlecraft, and other pieces of this sort, which are later used in shipyards to construct such vessels. In this manner, as few as 15 industrial replicators are enough to replicate the components needed to build a fleet of starships or to help a civilization recover from a planet-wide natural disaster.