In 1996 and 1997, Tajiri made several appearances for the World Wrestling Federation (WWF) under the name Yoshihiro Tajiri, as an enhancement talent. On an episode of Monday Night Raw in July 1997, he lost to Taka Michinoku, whom the WWF was pushing as its rising star in the Light Heavyweight division at the time. Tajiri's final appearance was a tag match with Brian Christopher, losing to Scott Taylor and Michinoku.
Tajiri joined Extreme Championship Wrestling in 1998. He made his television debut in ECW on the December 24, 1998 episode of Hardcore TV, clad in traditional blue and white trunks with an insignia of Japan's rising sun, and picked up a victory over Antifaz del Norte. He began a notable storyline rivalry with Super Crazy. The duo traded wins throughout 1999. His other notable single feuds included Little Guido and Jerry Lynn.
In early 1999, Tajiri began to emulate The Great Muta. His wrestling trunks were replaced with baggy Karate gi pants, and he began spraying green mist into the face of his opponents. He aligned himself with Steve Corino, who promised to teach him the American way and help him to integrate himself into United States culture. With Tajiri's notoriety increasing, he received an ECW World Heavyweight Championship title shot against Taz at Heat Wave, and on an episode on ECW on TNN, but failed to capture the title on both occasions. In 2000, he earned another shot at the championship against Justin Credible, but lost once again.
In early 2000, Taiji was recruited by Cyrus to join The Network group along with Steve Corino. During this time, Tajiri went up against Super Crazy for the ECW World Television Championship in a Japanese death match, a match with no disqualification and all weapons are legal. He lost, but gained the title back in a three-way dance involving Super Crazy and Little Guido. Shortly after becoming champion, Tajiri was asked by Cyrus to relinquish the belt to The Network so they could award the title to Rhino. Tajiri refused to turn the belt over and turned his back on the Network, becoming a babyface in the process. His first defense of the title ended in a no contest with Tommy Dreamer. Tajiri lost the title to Rhino on April 22. His rematch came on May 6, but he was unable to regain the title.
Tajiri would then form a tag team with Mikey Whipwreck, known as The Unholy Alliance, who were managed by The Sinister Minister. They entered the World Tag Team tournament and won the vacant ECW World Tag Team Championship. The team then started feuding with The Full Blooded Italians and lost the World Tag Team Championship to them only a day after they first won them. They failed to recapture them at Anarchy Rulz and again at November to Remember.
Shortly after his alliance with Whipwreck, Tajiri's amusing antics in the ring, stiff contact with opponents, and comedic backstage vignettes made him one of the most popular wrestlers in ECW. Tajiri remained loyal to the company until it folded in April 2001. He participated in ECW's final two shows: on January 12, where he picked up a victory over Super Crazy, and January 13, 2001, when he lost to Super Crazy.
Soon after the closure of ECW, Tajiri was hired by the World Wrestling Federation (WWF). Tajiri debuted on the May 24, 2001 episode of SmackDown! as a face. He was given the gimmick of WWF Commissioner William Regal's comedic assistant. With the help of Regal, Tajiri made his in-ring debut defeating Crash in a King of the Ring 2001 qualifying match. He was later eliminated by Rhyno. Tajiri would win his first championship in WWF by defeating X-Pac for the WWF Light Heavyweight Championship on an episode of Raw. X-Pac would quickly capture the WCW Cruiserweight Championship, and at SummerSlam, Tajiri would lose a championship unification match, therefore retiring the Light Heavyweight Championship. Tajiri would quickly bounce back as he began an on-screen relationship with Torrie Wilson and defeated Chris Kanyon for the WCW United States Championship. However, Tajiri would lose the belt to Rhyno soon afterward at Unforgiven in September. After Regal turned heel by joining The Alliance, Tajiri feuded with him throughout late 2001.
Tajiri won his first WCW Cruiserweight Championship on October 22, 2001, on Raw. The title became the WWF Cruiserweight Championship and replaced the WWF Light Heavyweight Championship when the WWF defeated The Alliance at Survivor Series on November 18. WWF was renamed World Wrestling Entertainment, and the roster was divided into two "brands": Raw and SmackDown!. Tajiri, along with Torrie Wilson, was drafted to SmackDown!, therefore making the Cruiserweight Championship a SmackDown! exclusive title. Tajiri went on to lose the title to Billy Kidman on an episode on SmackDown!. He would go on to beat Kidman at Backlash once again and won the Cruiserweight Championship again only 19 days after losing it.
He turned heel by forcing Wilson to dress like a geisha against her will. Tajiri would go on to lose the Cruiserweight Championship to The Hurricane in a triple threat match that also included Billy Kidman. Wilson eventually abandoned him during a Cruiserweight Championship match against The Hurricane; she did so by stripping on top of a table during the match that caused Tajiri to be counted out and later joined forces with Tajiri's rival, Maven. Tajiri became a face in WWE once more after Jamie Noble attacked him for touching Noble's girlfriend Nidia. This led to several Cruiserweight Championship matches between the two, including matches at No Mercy and Rebellion. He also participated in his first Royal Rumble match in January 2003.
Tajiri would leave the Cruiserweight division and moved to the tag team division, where he formed various tag teams, many of which were somewhat comical. These included Rey Mysterio, Funaki, and Rikishi. Five days prior to the 2003 Judgment Day show, Chavo Guerrero tore his biceps, forcing Eddie Guerrero to look for another partner. He chose Tajiri. They won the WWE Tag Team Championship at Judgment Day by defeating Team Angle in a ladder match. The following week, Guerrero and Tajiri managed to retain their titles by cheating. In addition, they also defeated Roddy Piper and his protégé Sean O'Haire at Madison Square Garden. After Guerrero and Tajiri lost the titles to Team Angle on the July 3 episode of SmackDown!, Guerrero turned on Tajiri, slamming him through the windshield of his lowrider truck. On the next SmackDown!, Guerrero explained that this was because, during the match, Tajiri had accidentally hit his lowrider. At SummerSlam, Tajiri competed in a fatal four-way match for the WWE United States Championship, but failed to win the title.
Tajiri would mark his return to the cruiserweight division by taking on the champion Rey Mysterio, and after losing, he spat green mist in Mysterio's eye, turning him heel once again. On September 23, Tajiri would beat Rey Mysterio for the WWE Cruiserweight Championship, which would be his third and last reign. Tajiri would successfully defend the championship in Mysterio's rematch at No Mercy after Akio and Sakoda, later known as Kyo Dai, interfered in the match. Tajiri would, however, lose the championship after 93 days to Mysterio on SmackDown!. Tajiri kept his heel persona going by spitting black mist in Nidia's eyes and "blinding" her (Nidia and Noble were both faces by this time.) Noble and Tajiri started a new feud, which quickly ended after Noble betrayed his blind girlfriend and turned heel again. At WrestleMania XX, Tajiri took part in a cruiserweight open for the Cruiserweight Championship, which was won by the reigning champion Chavo Guerrero.
In 2004, Tajiri had two chances to earn a shot at the WWE Championship. The first chance was in the 2004 Royal Rumble, where the winner would get a championship match at WrestleMania XX, but he was eliminated by Rhyno. His second chance was in a battle royal on SmackDown!, in which the winner would face then-champion Brock Lesnar at No Way Out. However, he lost once again when he was eliminated by the Big Show.
Following WrestleMania XX, Tajiri was drafted to Raw, where he turned face once again and feuded with the group of Eric Bischoff, Jonathan Coachman, and Garrison Cade. He would later feud with the Evolution stable for a good portion of 2004. Also that year, Tajiri formed a tag team with Rhyno. This team dissolved quietly after a failed attempt at winning the World Tag Team Championship from La Résistance at Unforgiven. On February 4, 2005, Tajiri reunited with William Regal to defeat La Résistance for the World Tag Team Championship on an episode of Raw emanating from Japan. After several defenses against La Résistance, among others, their reign ended on May 1 at Backlash, when they were eliminated from a tag team turmoil match by La Résistance.
Tajiri lost a three-way dance (that also featured Little Guido) to Super Crazy at ECW One Night Stand 2005 on June 12. Later that year, he formed a short-lived team with Eugene. In December 2005, Tajiri left WWE. His final televised WWE match was a loss to Gregory Helms on Heat. After the match, he received a standing ovation as a farewell. Tajiri returned for a one-night appearance at WWE's ECW One Night Stand 2006 pay-per-view, teaming with Super Crazy to lose to The Full Blooded Italians (F.B.I.).
On February 12, 2008, Tajiri made an appearance at a WWE house show in Tokyo, where he accompanied William Regal to the ring for a match against Ric Flair, which Regal lost.
On June 13, 2016, Tajiri was announced as a participant in WWE's Cruiserweight Classic tournament. The tournament kicked off on June 23 with Tajiri defeating Damian Slater in his first-round match. On July 14, Tajiri was eliminated from the tournament by Gran Metalik.
On December 3, at the NXT special at Osaka, Japan, Tajiri teamed with Akira Tozawa, losing to NXT Tag Team Champions DIY (Tommaso Ciampa and Johnny Gargano). On December 4, 2016, Tajiri announced he signed a full-time contract with WWE, and that he would make his return in early 2017. On the December 13 episode of 205 Live, vignettes for Tajiri's return began airing. On January 3, 2017, Tajiri debuted on 205 Live, defeating Sean Maluta. Following the match, The Brian Kendrick tried to welcome him back to the WWE, only for Tajiri to spit green mist onto his face. In January, Tajiri suffered a knee injury at an NXT taping. After weeks of recovery, Tajiri returned at a 205 Live taping in February, following a match between Lince Dorado and Brian Kendrick. On April 22, Tajiri officially announced his departure from the WWE.