Itami's signing to WWE was officially announced during an in-ring segment on July 12, 2014, in Osaka. He debuted on September 11 at NXT TakeOver: Fatal 4-Way, was introduced by William Regal, and fended off The Ascension when they interrupted the segment to confront Regal. Itami made his in-ring debut at the following day's tapings of NXT, defeating Justin Gabriel, after which he was attacked by The Ascension. During the following weeks, Itami kept engaging The Ascension in brawls, but continually lost due to the numbers disadvantage; the storyline was that as a newcomer in NXT, Itami did not have any friends in the locker room to help him. This continued until November, when Finn Bálor debuted as Itami's new partner. After weeks of feuding with The Ascension, Itami and Bálor defeated them at NXT TakeOver: R Evolution on December 11. Itami then participated in an NXT Championship number one contender's tournament, defeating Tyler Breeze in the first round, starting a feud between the two. He suffered his first defeat in NXT on January 15, 2015, when he was defeated by Bálor in the semi-finals, thus eliminating him from the tournament. At NXT TakeOver: Rival, Itami defeated Breeze in a rematch. They continued to exchange victories over each other until Itami lost a two-out-of-three falls match to Breeze on the April 1 episode of NXT to end the feud. On March 27, Itami won the WrestleMania Axxess NXT tournament by defeating Adrian Neville and then Finn Bálor to get a spot in the André the Giant Memorial Battle Royal on the pre-show of WrestleMania 31, where he was eliminated by eventual winner Big Show.
On May 20 at NXT TakeOver: Unstoppable, Itami was scheduled to face Finn Bálor and Tyler Breeze in a triple threat match to determine the number one contender for the NXT Championship; however, Itami was ruled out due to injury after an attack he suffered before the event started. In January 2016, it was reported that Itami had suffered complications with his shoulder and was "nowhere close to being able to return".
After over a year of inactivity, Itami returned to the ring on June 30, 2016, at an NXT live event, teaming with TM-61 to defeat Samoa Joe, Blake and Tino Sabbatelli in a six-man tag team match. On August 3, Itami made his televised return, defeating Cruiserweight Classic competitor Sean Maluta. At NXT TakeOver: Brooklyn II, Itami confronted Austin Aries, following the latter's match with No Way Jose, exchanging blows before attacking Aries with a GTS. On October 12 at an NXT live event, Itami suffered a neck injury after a botched powerslam from Riddick Moss and would not be able to team with Kota Ibushi in the Dusty Rhodes Tag Team Classic in which he was scheduled to compete in. In a video announcement published on October 18, Itami vowed to return to in-ring competition at NXT's first show in Japan on December 3. However, at the event, Itami did not compete but was instead in an in-ring segment alongside William Regal where they talked to the crowd.
Itami made his televised return on April 19, 2017, where he confronted NXT Champion Bobby Roode, before slapping him and executing a GTS. This led to a match at NXT TakeOver: Chicago, in which Itami was unsuccessful in winning the title from Roode. On the June 7 episode of NXT, furious after his loss to Roode, Itami faced Oney Lorcan, which ended in a no-contest after Itami performed three GTS's on Lorcan until Kassius Ohno attempted to save Lorcan, teasing a heel turn and a feud between the pair. On the July 5 episode of NXT, Itami teamed with Ohno in a tag team match against Sanity (Killian Dain and Alexander Wolfe), but lost after Itami refused to tag in Ohno. On the July 26 episode of NXT, Itami faced Ohno in the main event, which ended after Itami intentionally disqualified himself with a low blow to Ohno before viciously attacking and laying him out after two GTS's, followed by a third onto the steel steps, cementing his heel turn. On the August 2 edition of NXT, Itami cut an in-ring promo, demanding respect from the fans. He was interrupted by Aleister Black, who was scheduled to face Kyle O'Reilly in the night's main event. After a stare down between the two, Black hit Itami with Black Mass. Itami attempted to assault Black following the event, but the two were separated by fellow talent. At NXT TakeOver: Brooklyn III, Itami was defeated by Black. In his last NXT match, Itami defeated Fabian Aichner at a live event on December 16.
On November 21, 2017, it was announced that Itami would be moving to the main roster as part of the cruiserweight division on 205 Live. On the December 18 episode of Raw, Itami would make his main roster debut, aiding Finn Bálor to fend off and face Curtis Axel and Bo Dallas in a tag team match, which Itami and Bálor would win. On the December 25 episode of Raw, Itami would defeat The Brian Kendrick, in which Kendrick would suffer an injury following a Go to Sleep by Itami. This would lead to Itami entering a short feud with Kendrick's tag team partner, Gentleman Jack Gallagher, where Gallagher would challenge Itami to a match on the December 26 episode of 205 Live, which Itami would win. On the January 2, 2018, episode of 205 Live, Itami would be challenged to a rematch by Gallagher, however, Gallagher attacked Itami with an umbrella and a pipe before the match. On the January 23 episode of 205 Live, Itami would defeat Gallagher, ending their feud.
On the February 6 episode of 205 Live, Itami would be defeated by Roderick Strong in the first round of the WWE Cruiserweight Championship tournament. After that, Itami began teaming up with fellow Japanese wrestler Akira Tozawa, and they started a feud with Gran Metalik and Lince Dorado, which ended when they lost a tornado tag team match on the April 17 episode of 205 Live after Tozawa accidentally kicked Itami. After a tag team loss to The Brian Kendrick and Gentleman Jack Gallagher, Itami shoved Tozawa, signaling their break up. The following week on 205 Live, Itami defeated Tozawa to end the feud. On the June 5 episode of 205 Live, he attacked both Buddy Murphy and Mustafa Ali during their match, causing a disqualification and resulting in Itami being suspended from the following week's episode.
Soon after, he began a feud with Ali, Murphy, and Cedric Alexander for the Cruiserweight Championship. On the June 19 episode of 205 Live, Itami defeated Ali and Murphy in a triple threat match, thus earning a championship match. On the July 10 episode of 205 Live, Itami unsuccessfully challenged Alexander for the WWE Cruiserweight Championship. On the July 24 episode of 205 Live, Itami failed to win a number one contender's fatal four-way match also involving Ali, TJP, and the eventual winner Drew Gulak. On the August 7 episode of 205 Live, Itami defeated Ali in a singles match after which Ali collapsed. On the September 25 episode of 205 Live, Ali and Itami wrestled to a double count out. This led to a falls count anywhere match on the October 24 episode of 205 Live, which Itami lost.
Beginning in November 2018, Itami formed an alliance with Ariya Daivari. On the January 8, 2019 episode of 205 Live, Itami defeated Cedric Alexander to qualify for the WWE Cruiserweight Championship fatal four-way match involving champion Buddy Murphy, Kalisto, and Tozawa at the Royal Rumble, where he lost after being pinned by Murphy.
On January 29, it was reported that Itami requested and was granted his release from the company after a five year run. That same day, after a loss to Tozawa on 205 Live, Daivari turned on Itami. On February 22, he was officially released by WWE.