On March 31, 2016, Tozawa was announced as a participant in WWE's Global Cruiserweight Series tournament, which was later renamed the "Cruiserweight Classic". The tournament began on June 23 with Tozawa defeating Kenneth Johnson in his first round match. On July 14, Tozawa defeated Jack Gallagher in his second round match. On August 26, Tozawa was eliminated from the tournament in the quarterfinals by Gran Metalik.
On the September 5 episode of Raw, Tozawa was announced as part of WWE's upcoming cruiserweight division. On November 2, Tozawa was announced as a roster member for the upcoming cruiserweight-centric 205 Live show. On February 6, 2017, Tozawa made his debut on Raw, defeating Drew Gulak. In June, Tozawa was recruited by Titus O'Neil to the "Titus Worldwide" stable and entered in a feud with the Cruiserweight Champion Neville. At the Great Balls of Fire event, Tozawa was unsuccessful in his first championship opportunity against Neville; however, he pinned Neville in a tag team match the next night. On the August 14 episode of Raw, Tozawa defeated Neville to win the Cruiserweight Championship. However, at SummerSlam, he dropped the title to Neville, ending the reign at 6 days. In late 2017, Tozawa quietly left Titus Worldwide.
On the October 10 episode of 205 Live, Tozawa was attacked on the ramp by Drew Gulak, which began a rivalry. Tozawa was out of action for a few weeks until he returned on the October 31 episode of 205 Live, where he defeated Gulak. On the November 21 episode of 205 Live, Tozawa defeated Gulak in a Street Fight, ending their feud. In February 2018, Tozawa formed a tag team with Hideo Itami, and they defeated Gran Metalik and Lince Dorado on the March 13 episode of 205 Live, but got into a confrontation with them and their ally Kalisto post-match. After trading victories between each other, they finally lost a tornado tag team match to Metalik and Dorado on the April 17 episode of 205 Live after Tozawa accidentally kicked Itami. On the May 1 episode of 205 Live, Tozawa and Itami were defeated by The Brian Kendrick and Gentleman Jack Gallagher after another miscommunication between the two, after which Itami attacked Tozawa, thus ending their partnership. The following week, Tozawa was defeated by Itami.
On the January 2, 2019, episode of 205 Live, he defeated Drew Gulak to qualify for the fatal four-way match at Royal Rumble for the Cruiserweight Championship, which was won by champion Buddy Murphy. On the February 14, 2019, episode of 205 Live, Tozawa defeated Cedric Alexander, Humberto Carrillo and Lio Rush to become the number one contender for the Cruiserweight Championship at the Elimination Chamber, where he was defeated by Murphy.
On October 14, 2019, as part of the 2019 WWE Draft, Tozawa was drafted to the Raw brand. He made his debut on the November 18th edition of Raw, where he was defeated by Buddy Murphy. At Survivor Series, he unsuccessfully challenged for Lio Rush's Cruiserweight Championship in a triple threat match, also involving Kalisto. Three days later on the November 27th episode of NXT, Tozawa had a singles match with Rush for the Cruiserweight Title, as Rush had pinned Kalisto at Survivor Series. However, he failed to win the title. Tozawa continued appearing on Raw, losing to the likes of Drew McIntyre and Aleister Black. On the December 16 episode of Raw, he competed in a #1 Contender's Gauntlet match for the WWE United States Championship, where he was able to eliminate R-Truth but was eliminated by Ricochet.
On the December 23 episode of Raw, Tozawa ambushed R-Truth outside Rockefeller Center to pin him and become the new 24/7 Champion. Later on in the night, Tozawa lost the title to a man impersonating Santa Claus. When R-Truth and Tozawa caught up to Santa, R-Truth pinned Santa to win back the title. On April 12, 2020, Tozawa was announced as a participant in the Interim Cruiserweight Title tournament, representing Group B in the tournament. He defeated Isaiah "Swerve" Scott in his first match and Gentleman Jack Gallagher in his second match, but lost to El Hijo del Fantasma, thus leaving him with two wins in the tournament failing to advance to the finals. At Backlash, Tozawa appeared during a segment between Street Profits and The Viking Raiders with a gang of ninjas on motorcycles and acting as a heel. The Street Profits and The Viking Raiders were able to fend off Tozawa's gang until Tozawa introduced a giant sword-wielding ninja which led to The Viking Raiders and The Street Profits fleeing. The following night on Raw, Tozawa and his gang of ninjas faced The Street Profits and The Viking Raiders in an eight-man tag team match in a losing effort. On the June 22 episode of Raw, Tozawa defeated Truth with the help of Bobby Lashley to become the 24/7 Champion for a second time, only to lose it back to Truth one week later. On the August 3 episode of Monday Night Raw, Tozawa won the 24/7 Title for a third time in a triple threat match against champion Shelton Benjamin and R-Truth. A week later R-Truth, disguised as a Tozawa's ninjas, pinned him ending his third reign. He would win the title five other times. On the February 15, 2021 episode of Raw, he pinned R-Truth backstage to win the 24/7 Championship, but lost it to Bad Bunny after being attacked by Damian Priest, ending his reign at 28 seconds. When Reggie won the title after he was abandoned by Nia Jax and Shayna Baszler, he and R-Truth began feuding with him, even building a temporary alliance with Drake Maverick. On the September 27 episode of Raw, Tozawa would be defeated by Keith "Bearcat" Lee.
In early 2022, after spending months with both him and Tamina Snuka trying to win the 24/7 Title, Tozawa and Tamina ended up becoming lovers. On the March 28 episode of Raw, Tozawa and Tamina became engaged. On April 18, Tozawa won the championship for the fifth time by pinning his partner, Tamina, during their wedding ceremony. Tozawa then lost the championship back to Dana Brooke. On the November 17, 2022 episode of Raw, Tozawa turned face after beating Baron Corbin and JBL in a game of cards. He would subsequently begin a cosmetic change, returning to wear more traditional ring gear as opposed to his ninja gi and dying his hair blonde.
Tozawa joined Alpha Academy and was assigned as a "junior member" of the stable on the October 23, 2023 episode of Raw. In a bid to prove himself to the stable, Tozawa challenged Bronson Reed to a match but lost. The following day, Tozawa appeared on night one of NXT Halloween Havoc and stole Noam Dar's NXT Heritage Cup. Tozawa would return the Cup to Dar on night two before managing to spook Dar into a match for the Cup on the following week's NXT but lost the match 2-1 under British Round Rules.
In April 2024, Alpha Academy leader Chad Gable turned heel after failing to win the Intercontinental Championship multiple times, with Gable assigning blame that he had spent too much time training his "loser" stablemates and declaring that the stable's focus is to win Gable the Intercontinental Championship. After weeks of mistreatment by Gable on the rest of the stable for not assisting him to win the title by cheating, Tozawa, alongside stablemates Otis and Maxxine Dupri, walked out of Alpha Academy on the June 17 episode of Raw.