Here is every character from Star Trek: The Next Generation who also appeared in Star Trek: Deep Space Nine. Debuting in 1992, DS9 was the first spinoff from TNG and centered on the eponymous space station commanded by Captain Benjamin Sisko (Avery Brooks) but it included one TNG regular, Chief Miles O'Brien (Colm Meaney), in its core cast. In addition, when Michelle Forbes declined to play TNG's Ro Laren on DS9, her character was replaced by a new Bajoran, Major Kira Nerys (Nana Visitor).
Star Trek: Deep Space Nine premiered while TNG was still on the air and then became the flagship Star Trek series after The Next Generation cast moved onto feature films. DS9 ended its seven-season run in 1999. DS9 wasn't as popular as TNG, nor was it the same kind of ratings juggernaut its predecessor was. In the first couple of seasons, DS9's producers struggled to establish what the tone and direction of the spinoff would be, while many Star Trek fans rejected DS9's premise of being set on a space station, instead of a starship exploring strange new worlds.
To help out DS9's ratings and raise its profile, several familiar faces from TNG, starting with Captain Jean-Luc Picard (Patrick Stewart) himself, made appearances in the spinoff, especially in its early seasons. This also helped establish DS9's growing prominence in the Star Trek universe. By season 4, DS9 had firmly figured out its characters and the series' identity, especially after the addition of TNG's Worf (Michael Dorn) to the cast as a series regular.
In DS9's later seasons, TNG guest stars became more infrequent and happened organically, as opposed to stunts designed to boost ratings for the fledgling series. Naturally, TNG would still occasionally be mentioned, such as when Chief O'Brien and Worf swapped stories from the U.S.S. Enterprise-D about Reginald Barclay (Dwight Schultz) and Commander Geordi La Forge (LeVar Burton) over a bottle of blood wine. DS9 also resolved the storylines of many of its TNG guest stars, especially on the Klingon side of things, thanks to Worf.
Miles & Keiko O'Brien
Chief Miles O'Brien operated the Enterprise-D's transporter and he married Professor Keiko Ishikawa (Rosalind Chao) in TNG. The O'Briens then moved to Deep Space Nine, where Keiko taught at the station's school and Miles became DS9's Chief Engineer. Chief O'Brien was a series regular and appeared in all seven seasons of DS9 as a key member of the station's senior staff. After the series ended, Miles became a Professor of Engineering at Starfleet Academy.
Worf
Lieutenant Commander Worf was the Chief of Security on the U.S.S. Enterprise-D and he joined Star Trek: Deep Space Nine as a series regular starting in season 4's premiere, "The Way of the Warrior". (He also visited DS9 in the TNG episode "Birthright, Part 1". Worf's colorful Starfleet career continued on DS9, where he became the station's Strategic Operations Officer, commanded the U.S.S. Defiant, but he also served alongside his close friend, Klingon General Martok (H.G. Hertzler). The Klingon married Jadzia Dax (Terry Farrell) in DS9 season 6; after she died and at the conclusion of the Dominion War, Worf became the Federation Ambassador to the Klingon homeworld. Worf also did double duty and appeared in every TNG feature film and, in Star Trek: Picard's era, Worf became Captain of the Enterprise-E after Jean-Luc Picard.
Captain Jean-Luc Picard
Captain Picard appeared in DS9's series premiere, "Emissary", where he charged Commander Sisko with his mission to bring Bajor into the Federation. Sisko, whose wife Jennifer was killed in the Battle of Wolf 359 in TNG, when Picard was assimilated by the Borg and became their avatar, Locutus, was openly hostile towards Picard. The Captain of the Enterprise, along with Dr. Beverly Crusher (Gates McFadden), later visited DS9 in the TNG episode "Birthright, Part 1", but Picard didn't run into Sisko.
Lursa and B'Etor
The Duras sisters, Lursa (Barbara March) and B'Etor (Gwyneth Walsh), were popular Klingon villains who were introduced as enemies of Worf in the TNG episode "Redemption". Lursa and B'Etor appeared in the DS9 season 1 episode "Past Prologue", where they conspired with a Bajoran terrorist who was planning to build a bomb. The Duras sisters later met their demise in Star Trek Generations.
Q and Vash
Vash (Jennifer Hetrick) was a profiteer and a former love interest of Captain Picard, while Q (John de Lancie) is a flamboyant omnipotent being and frequent nemesis of Picard who debuted in TNG's series premiere, "Encounter at Farpoint". Both Vash and Q appeared in the DS9 season 1 episode, "Q-Less", where Vash tried to elude Q on the station while selling artifacts from the Gamma Quadrant. "Q-Less" is most famous for how it contrasted how Sisko dealt with Q compared to Picard, with DS9's Commander punching the omnipotent being and Q gasping, "You hit me! Picard never hit me!" To which Sisko retorted, "I am not Picard!"
Lwaxana Troi
Lwaxana Troi (Majel Barrett-Roddenberry) was the mother of TNG's Counselor Deanna Troi (Marina Sirtis) and she was a telepathic Betazoid always on the hunt for a mate. Lwaxana made three appearances in DS9; in season 1's episode "The Forsaken", Mrs. Troi became romantically interested in Constable Odo (Rene Auberjonois). Lwaxanaa returned in DS9 season 3's episode, "Fascination", where she suffered from Zanthi fever, an illness that made everyone around her amorous due to her telepathy. In DS9 season 4's "The Muse", Odo married a pregnant Lwaxana to free her and her child from her previous marriage. "The Muse" was also Majel Barrett-Roddenberry's final appearance as Lwaxana Troi.
Admiral Nechayev
Admiral Alynna Nechayev (Natalia Nogulich) was one of Captain Picard's Starfleet superior officers who was a recurring presence in TNG. She specialized in the Cardassian-Bajoran-Maquis conflict. Nechayev appeared in the DS9 season 2 two-part episode "The Maquis" where she orders Commander Sisko to negotiate with the Maquis. Admiral Nechayev returned in the DS9 season 3 two-parter, "The Search", where she appeared in a simulation used by the Vorta to anticipate how Deep Space Nine would react to increased Dominion presence the Alpha Quadrant.
Gowron
Gowron (Robert O'Reilly) was the High Chancellor of the Klingon Empire and he alternated between being a friend and foe of Worf's. Gowron made numerous appearances in DS9, starting in the season 3 episode, "The House of Quark". In DS9's season 5 premiere, "Apocalypse Rising", Gowron was suspected of being replaced by a changeling. Worf finally killed Gowron in one-on-one combat in the DS9 season 7 episode "Tacking Into The Wind" and succeeded him as Chancellor before passing on the leadership of the High Council to General Martok.
Thomas Riker
The result of a transporter accident, Thomas Riker (Jonathan Frakes) is the exact duplicate of Commander Will Riker. At one point, TNG's writers even planned to kill Will and replace him with Thomas on the Enterprise. Instead, Thomas transferred to the U.S.S. Gandhi but sometime between TNG season 6 and DS9 season 3, Riker left Starfleet and joined the Maquis. Thomas appeared in the DS9 episode "Defiant", where he kidnapped Major Kira and stole the U.S.S. Defiant for the Maquis. Thomas eventually surrendered to the Cardassians and was imprisoned; he hasn't been seen since in Star Trek canon.
Toral
Toral (Rick Pasqualone) was the illegitimate son of Duras and the nephew of Lursa and B'Etor, who debuted in TNG's "Redemption" two-part story. In the DS9 season 4 episode "The Sword of Kahless", Toral tried and failed to steal the fabled Klingon Bat'leth from Worf, Jadzia Dax, and the Dahar Master Kor (John Colicos).
Kurn
Kurn (Tony Todd) was Worf's tragic brother, who was originally introduced in TNG. In the DS9 season 4 episode "Sons of Mogh", Kurn arrives on the station and asks Worf to help him escape the dishonor that has befallen the House of Mogh, which was the result of Worf's rejecting the Klingons' war with Cardassia. The depressed Kurn asks Worf to ritually kill him according to Klingon custom. Instead, Worf asks Dr. Bashir to wipe Kurn's mind; now known as "Rodek", Kurn joins a family friend of the House of Mogh with no memory of his true identity.
Alexander Rozhenko
Alexander Rozhenko is the long-suffering son of Worf and Ambassador K'Ehleyr (Suzie Plakson); in TNG, he was a young boy played by Brian Bonsall but in DS9, the older Alexander was played by Marc Worden. Alexander appeared in the DS9 season 6 episode "Sons and Daughters", where he struggled to be a Klingon Warrior serving aboard a Bird-of-Prey as part of the House of Martok. Alexander returned for the wedding of his father and Jadzia Dax in the Star Trek: Deep Space Nine season 6 episode, "You Are Cordially Invited".