Deep Space Nine (DS9; previously Terok Nor) is a fictional space station, the eponymous primary setting of the American science fiction television series Star Trek: Deep Space Nine which aired from 1993 to 1999. It serves as a base for the exploration of the Gamma Quadrant via the Bajoran wormhole and is a hub of trade and travel for the sector's denizens. It is run by a joint crew of Starfleet and Bajoran officers and it is the home port of a number of Starfleet runabouts, as well as the starship USS Defiant.

The station is featured in the opening for all 176 episodes of Deep Space Nine, as well as part one of the Star Trek: The Next Generation episode "Birthright", the first Star Trek: Voyager episode "Caretaker", and the Star Trek: Lower Decks episode "Hear All, Trust Nothing". Many story arcs introduced on Next Generation are extended by events that occur on the station. The station builds on the legacy of the Cardassian-Federation interactions introduced by that show and the first episode, "Emissary", includes a visit from the Enterprise-D. Deep Space Nine provides the location for the crew to interact with the characters and elements established by the Star Trek universe, and introduces additional elements.

The station exists in a time and place also occupied by The Next Generation, Voyager, Lower Decks and the seventh through tenth films, the 2370s through early 2380s. This is itself part of the bigger Star Trek universe, which was initiated by the original Star Trek TV show of the late 1960s, created by Gene Roddenberry.

For the show's conceptual design of Deep Space Nine, it is over a kilometer in diameter, is composed of a broad outer docking ring; an inner habitat ring containing residential apartments; and a central core containing the promenade, the fusion reactors, and the operations center, known as "Ops". Three sets of docking pylons sweep up and down equidistantly around the docking ring, defining an almost spherical shape. It is of identical design to another Cardassian station, Empok Nor. Originally situated in orbit of Bajor, the station was moved in the series' pilot episode to a position at the mouth of the newly discovered wormhole, where it would remain throughout the remainder of the series. This made the station three hours away from Bajor by shuttle, as described in three episodes, although it was mentioned as being five hours away in another episode, (presumably as Bajor was in far orbit of its sun relative to DS9's position). However, it would appear that the journey could be completed in a much shorter length of time if necessary by engaging warp drive within the Bajoran star system.

The promenade is the main public thoroughfare in which visitors and residents congregate. Common locations on the promenade depicted or mentioned in Star Trek: Deep Space Nine include Quark's Bar (the crew's most frequently seen off-hours location, which also includes holosuites), the infirmary, the replimat (a self-service replicator eatery), a Bajoran temple, Elim Garak's clothing shop, chief of security Odo's office, a Klingon restaurant, a mineral assay office, a candy kiosk, and, for a brief time during the first season, Keiko O'Brien's classroom. There are normally about 300 permanent residents of the station, though the station can accommodate up to seven thousand.

In the Star Trek universe, Deep Space Nine was originally a Cardassian mining and refinery station named Terok Nor in orbit around Bajor, and it was built by Bajoran slave labor under Cardassian rule in 2346. The station was commanded by Gul Dukat, the last Cardassian prefect of Bajor. It was abandoned toward the end of the Cardassian occupation of Bajor in 2369; the Cardassians stripped items of value on their way out. The station had some major problems, including a trap left by the Bajoran resistance that nearly killed everyone on the station when it was accidentally activated by the new tenants.

In 2369, the Bajorans assumed control of the station. The Bajoran provisional government requested the assistance of the United Federation of Planets, who renamed it Deep Space Nine (DS9) and placed Commander Benjamin Sisko in command. Despite the Federation's presence, the station remains subject to Bajoran law. Shortly after Sisko assumed command, his crew discovered the Bajoran wormhole. The station was moved to the mouth of the wormhole to secure Bajor's claim to it, after which the station assumed great commercial, scientific and strategic importance.

After the encounter with the Dominion in 2370, in which the Galaxy-class USS Odyssey was destroyed by just three Dominion vessels and owing to other threats, it became obvious that the station's existing occupation-era weapons would be woefully inadequate defending the station. The station's weapons and defensive systems were later heavily upgraded. The upgrades included the installation of multiple rotary torpedo launchers on the docking pylons and weapons sail towers, stowable phaser banks that retract when not in use and improved shields that extend 300 meters around the station. The total torpedo inventory was increased to 5,000.