On August 22, 2015, Asuka attended WWE's NXT Takeover: Brooklyn event in Brooklyn, New York, where she was shown on camera and identified as "Kanna". On August 27, it was confirmed that she had signed with WWE a few weeks earlier. "Kanna" appeared at the September 10 NXT tapings, where she announced that her new ring name was "Asuka". Her debut appearance, which aired on September 23, ended with her being confronted by Dana Brooke and Emma, which led to her debut match on October 7 at NXT Takeover: Respect, where she defeated Brooke. After her debut, Asuka continued racking victories over various competitors, including main roster Diva Cameron. Asuka continued her feud with Brooke and Emma, which led to a match between Asuka and Emma, at NXT Takeover: London on December 16, where Asuka was once again victorious.
On the January 13, 2016, episode of NXT, Asuka competed in a number one contender's over-the-top-rope battle royal for Bayley's NXT Women's Championship, which was won by Carmella. On the February 10 episode, Asuka saved Carmella from Eva Marie and Nia Jax after her loss to NXT Women's Champion Bayley. Afterwards, Asuka had a staredown with Bayley, indicating that she wanted a shot at her title. After Asuka and Bayley defeated Marie and Jax in a tag team match on the March 16 episode, an NXT Women's Championship match between the two was made official by William Regal. On April 1 at NXT TakeOver: Dallas, Asuka defeated Bayley to win the NXT Women's Championship, becoming the first Japanese holder of the title. On June 8 at NXT Takeover: The End, she successfully defended the title against Nia Jax. After Bayley earned herself a title rematch against Asuka, tensions between the two began to rise, leading to the match on August 20 at NXT Takeover: Brooklyn II, where Asuka retained the title. On November 19 at NXT Takeover: Toronto, Asuka successfully defended the NXT Women's Championship against Mickie James. Following her victory, Asuka refused to shake James' hand, beginning Asuka's heel turn.
On the January 11, 2017 edition of NXT, Asuka was attacked by Billie Kay and Peyton Royce until Nikki Cross appeared to save her, only to later attack Asuka. This prompted Asuka to demand to face all three women for her title, with William Regal granting the request. At NXT TakeOver: San Antonio, Asuka retained her title against Cross, Kay, and Royce, and in February, she became the longest reigning champion on NXT, male or female. Asuka's next title opponent was the then-undefeated Ember Moon, who she faced at NXT TakeOver: Orlando on April 1. Asuka continued her villainous persona during the match, which included shoving the referee into Moon to avoid Moon's finisher, the Eclipse, en route to successfully defending the title.
On the May 3 episode of NXT, Asuka turned heel and interfered in a number one contender's battle royal by attacking Ember Moon, Nikki Cross and Ruby Riot, the last three competitors, causing the match to be ruled a no contest. Afterwards, it was announced that Asuka would defend the NXT Women's Championship in a fatal four way match against Moon, Cross and Riot at NXT TakeOver: Chicago, though Moon was later pulled from the match due to a legitimate shoulder injury. At the event on May 20, the evil Asuka defeated Riot and Cross to continue her title reign. On the June 14 NXT, Asuka defended the title in a three-way elimination match, which ended in a no contest when she and Nikki Cross brawled backstage, after Riot had been eliminated. This led to a Last Woman Standing match two weeks later, where Asuka defeated Cross to retain the NXT Women's Championship. Two days later, Asuka defeated Ember Moon at NXT TakeOver: Brooklyn III to continue her title reign. On August 21, WWE announced that Asuka had broken her right collarbone during the match after landing awkwardly from a throw. She was expected to miss six to eight weeks. Due to her injury, Asuka relinquished the NXT Women's Championship at the August 24 NXT tapings, ending her reign at 510 days. William Regal, after calling her "one of the greatest champions in WWE history", announced during her farewell segment that negotiations had begun to move Asuka up to WWE's main roster.