Jax was signed by WWE in early 2014 and was assigned to their developmental territory NXT, based at the WWE Performance Center in Orlando, Florida. She made her in-ring debut on May 7, 2015 at an NXT house show under the ring name Zada, where she and Devin Taylor were defeated by Bayley and Carmella in a tag team match. In August, her ring name was changed to Nia Jax.
After a series of introductory vignettes throughout September, Jax made her debut on October 14, 2015 episode of NXT, defeating Evie in a singles match. Following her debut, she was shown slowly turning into a villainess while going on a winning streak, defeating various enhancement talents and Carmella. In November, Jax formed an alliance with Eva Marie and started her first feud with Bayley over the NXT Women's Championship, cementing Jax's heel persona. Jax earned a title match on December 16 at NXT TakeOver: London, but she was unsuccessful in winning the title.
On the January 27, 2016 episode of NXT, Jax returned to the ring after a month of inactivity due to a storyline injury with a win over Liv Morgan. On the February 10 episode of NXT, Jax assisted Marie in an attack on Bayley and Carmella after their championship match until Asuka made the save, which led to a tag team match on February 24 episode of NXT, where Jax and Marie scored the victory. On the 25 May episode of NXT, Jax won a triple threat match over Alexa Bliss and Carmella to become the number one contender to Asuka's NXT Women's Championship. On June 8, Jax lost her title match at NXT TakeOver: The End. On the July 20 episode of NXT, Jax was defeated by Bayley. On the December 28 episode of NXT, Jax lost to Asuka for the NXT Women's Championship in her last match in NXT.
Shortly after being drafted from NXT to the Raw brand as part of the 2016 WWE draft, which took place on July 19, 2016, Jax started a winning streak against various competitors. In September, Jax was placed in her first feud with Alicia Fox, which led to a match between the two on September 12 episode of Raw that ended in a no contest after she performed a spear on Fox through the ringside barricade. In a rematch between the two which took place on the Clash of Champions pre-show on September 25, Jax defeated Fox via pinfall. On November 20 at Survivor Series, Jax was a part of Team Raw in a five-on-five elimination tag team match and was eliminated via submission by Becky Lynch.
In December, Jax started a feud with Sasha Banks, whom she injured as part of the storyline. This led to two matches between the two: Jax was victorious on the Royal Rumble pre-show on January 29, 2017, but she lost on March 5 at Fastlane. On April 3 at WrestleMania 33, Jax competed in a fatal four-way elimination match against Banks, Bayley and Charlotte Flair for the Raw Women's Championship, but she was eliminated first. On the June 26 episode of Raw, Jax competed in a gauntlet match to determine a number one contender for Alexa Bliss's Raw Women's Championship, eliminating Bayley, Mickie James, Dana Brooke and Emma consecutively before being defeated by Banks. On September 24 at No Mercy, Jax took part of a fatal five-way match for the Raw Women's Championship, but she failed to win the title. On November 19 at Survivor Series, Jax once again was a part of Team Raw in a five-on-five elimination tag team match from which she was eliminated by countout.
Starting from December 4 episode of Raw, Jax was placed in an on-screen relationship with then-WWE Cruiserweight Champion Enzo Amore, which was dropped when Amore was released from the WWE in January 2018. On January 28 at the Royal Rumble, Jax participated in the first women's royal rumble match where she entered at number 22 and eliminated Jacqueline, Kelly Kelly, Ruby Riott and Naomi before being eliminated by six women. Throughout January and February, Jax started a feud with the undefeated Asuka, whom she failed to defeat on February 25 at Elimination Chamber and a week later on Raw.
In March, Jax turned face for the first time in her career after being insulted and betrayed by former best friend, Raw Women's Champion Alexa Bliss, who body shamed her and made fun of her personality. This led to a title match between the two on April 8, 2018, at WrestleMania 34, where she won the match to capture the Raw Women's Championship, marking the first title win of her career. In a rematch between the two, that took place a month later at Backlash on May 6, Jax went onto defeat Bliss and retain her title. In May, Jax started a feud with Ronda Rousey, whom she challenged to a title match at Money in the Bank pay–per–view. During that feud, Jax briefly returned to her villainous theatrics. At Money in the Bank, on June 17, Jax lost her championship back to Bliss, who interfered in her match and attacked both Jax and Rousey, cashing in her Money in the Bank contract (which she won earlier that night). A month later, on July 15, Jax invoked her rematch clause at Extreme Rules, but was unsuccessful in regaining the title in an extreme rules match.
After a short hiatus due to a minor injury, Jax returned on the September 17 episode of Raw, as the mystery tag team partner of Ember Moon and the two went on to defeat Alicia Fox and Mickie James. One month later, on October 28, at the WWE Evolution pay–per–view, Jax eliminated Moon as she won a battle royal to earn a future women's championship match. On the November 5 episode of Raw, shortly after defeating Moon in a singles match, Jax once again turned heel as she aligned herself with Tamina (with whom she had multiple confrontations the past few weeks) and attacked Moon. On November 18, at Survivor Series, Jax was the sole survivor for Team Raw against Team SmackDown in an interbrand women's traditional five–on–five tag team elimination match. The following month, on December 16, at the TLC: Tables, Ladders & Chairs event, Jax failed to regain the Raw Women's Championship from Rousey.
On January 27, 2019, at the Royal Rumble event, Jax competed in the women's Royal Rumble match, entering at number 29 and lasting for over 11 minutes before she was eliminated by Becky Lynch. Later that night, Jax made history as she became the first person to ever compete in two Royal Rumble matches on the same night and the fourth woman to ever compete in a men's Royal Rumble match, after she attacked R-Truth and took his number 30 spot. Despite eliminating Mustafa Ali, Jax was ganged up by Dolph Ziggler, Rey Mysterio and Randy Orton with their finishers, before being eliminated by Mysterio.
On February 17,2019 Jax and Tamina competed in a six–team Elimination Chamber match at the namesake pay–per–view to determine the inaugural holders of the WWE Women's Tag Team Championship, where they eliminated two teams before being eliminated by two teams. In April, at WrestleMania 35, Jax and Tamina took part of a fatal four–way match for the Women's Tag Team Championship, however, the match was won by The IIconics (Billie Kay and Peyton Royce). Shortly after, it was announced that Jax had suffered ACL tears on both of her knees and would require surgery to repair them which would sideline her from in–ring competition for at least nine months.
Nia Jax returned from her injury on the April 6, 2020 edition of Raw, the night after WrestleMania 36, defeating Deonna Purrazzo in a squash match. On the May 25 episode of Raw, Jax defeated Charlotte Flair and Natalya in a triple threat match to challenge Asuka for the Raw Women's Championship at Backlash. At Backlash, Asuka and Jax wrestled to a double countout, therefore Asuka retained the title. The next night on Raw, Jax challenged Asuka again for the title. During the match, Jax became upset with the referee and pushed him down. The referee was going to disqualify Jax, however, Asuka rolled up the distracted Jax for a fast count to retain the title. Nia Jax was then suspended indefinitely from WWE for attacking a WWE official on the August 3, 2020 episode of Raw. Despite her suspension, she made her appearance on August 17, 2020 episode of Raw, brawling with Shayna Baszler during her tag team match with Asuka against Sasha Banks and Bayley. It was then announced that Bayley and Banks would defend the Women’s Tag Team Championships against Jax and Baszler at Payback, thus lifting Jax’s suspension. At Payback, Jax and Baszler would win both the match and the titles, giving Jax her first tag team championship. On December 20, at TLC, Baszler and Jax lost the titles to Asuka and Charlotte Flair. On January 31, Baszler and Jax became record-tying two-time WWE Women's Tag Team Champions, defeating Charlotte Flair and Asuka on the Royal Rumble kickoff show due to an interference from Lacey Evans and Ric Flair. On the May 14 edition of SmackDown, they lost the titles to Natalya and Tamina.
After weeks of problems between the two, on the September 20 episode of Raw, her alliance with Shayna Baszler ended when she was brutally attacked by Baszler after their match against each other. On November 4, 2021, Jax was released from her WWE contract.
Jax returned in the 2023 Women's Royal Rumble as the number thirty spot, and was eliminated by the combined efforts of the remaining eleven women in the ring. She made her full-time return to WWE on the September 11 Raw, attacking Raquel Rodriguez and Rhea Ripley. On the September 25 episode of Raw, Jax had her first match back where she defeated Zoey Stark. At Crown Jewel, Jax failed to win the Women's World Championship from Ripley in a fatal-five way match also involving Rodriguez, Stark, and Shayna Bazler, with Ripley pinning Baszler to retain the title. Following Crown Jewel, Jax began a winning streak by defeating Baszler, Rodriguez, and Stark in singles competition. Jax would then engage in a feud with Becky Lynch, reigniting their history from 2018 when Jax gave Lynch a legit concussion and broken nose. This led to a match between the two at the Day 1 edition of Raw on January 1, 2024, where Jax was victorious. In the 2024 women’s Royal Rumble match, Jax entered at number nineteen and eliminated eight superstars, thus tying the record for the most eliminations by a superstar in a women’s Royal Rumble match, before being eliminated by the debuting Jade Cargill. Jax next challenged for Rhea Ripley's Women's World Championship in the main event of Elimination Chamber: Perth, where she was defeated by Ripley.
During night one of the 2024 WWE Draft held on April 26, Jax was drafted to WWE SmackDown. Jax then began participating in the 2024 Queen of the Ring tournament, defeating Naomi in the first round, Jade Cargill in the quarterfinals by disqualification, Bianca Belair in the semifinals, and Lyra Valkyria in the finals at King and Queen of the Ring to win the tournament, earning a WWE Women's Championship match at SummerSlam. At the event on August 3, Jax defeated Bayley to win her second WWE Women's Championship. On the August 30 episode of SmackDown, Jax defeated Michin to retain her title in a street fight in her first title defense. At Bad Blood on October 5, Jax retained her title against Bayley after interference from Stratton. At Crown Jewel on November 2, Jax faced Women's World Champion Liv Morgan for the inaugural WWE Women's Crown Jewel Championship in a losing effort after interference from Morgan's Judgment Day stablemates, "Dirty" Dominik Mysterio and Raquel Rodriguez.