On the August 29, 2008 episode of SmackDown, Nikki's twin sister Brie debuted and defeated Victoria. She quickly began a scripted rivalry with Victoria and Victoria's accomplice, Natalya, and had a series of matches against them. In each match, Brie would roll out of the ring and go underneath it, emerging and appearing revived, and then win the match. On the October 31 episode of SmackDown, when Brie went under the ring Victoria grabbed her legs, but a second pair of legs kicked her off, implying that a second person was under the ring. The following week on SmackDown, Brie picked up a win against Victoria and then ran under the ring to escape Natalya and Victoria, but Victoria and Natalya both reached for Brie under the ring, resulting in both Nikki and Brie being pulled out. The twins then attacked them and celebrated afterward. The twins had their first official match as a team on the November 21 episode of SmackDown, defeating Victoria and Natalya. They continued competing in tag team matches over the following months.
Starting in November, the twins developed an on-screen relationship with The Colóns (Carlito and Primo), appearing in numerous backstage segments with them, and accompanying them to the ring. In February 2009, the storyline expanded to include John Morrison and The Miz, who flirted with the Bellas and took them on a date for Valentine's Day. The date provoked a rivalry between the teams of The Miz and Morrison and Primo and Carlito, with the four competing for the affection of the twins, who were seemingly unable to choose between them. On March 17, on ECW, Carlito and Primo, aiming for Morrison and The Miz, accidentally spat apples in the face of Brie. Nikki began to laugh at Brie's misfortune, and a fight broke out between the two, which led to Nikki leaving with The Miz and Morrison, while Brie stayed with Primo and Carlito, with Nikki turning heel in the process. Brie won her first match over Nikki in a six-person intergender tag team match on SmackDown the following week. On ECW on March 31, the villainous Nikki pinned Brie in their first singles match against each other, after a distraction from Morrison and The Miz.
On April 15, 2009, The Bella Twins were both drafted to the Raw brand as part of the 2009 supplemental draft. On April 27, Brie made her Raw in-ring debut in an eight-Diva tag team match, which her team won. Nikki also made an appearance, reuniting with her twin, as she was under the ring to help Brie during the match, turning face in the process. Nikki then made her in-ring debut for the brand the following month in a battle royal but was eliminated by Beth Phoenix.
On June 29, 2009, they were both traded to the ECW brand. They debuted on ECW the following night on The Abraham Washington Show, as the special guests. They quickly developed a storyline feud with Katie Lea Burchill, when Nikki defeated her in a match by switching places with Brie behind the referee's back. The following week on Superstars, Brie defeated Burchill after a similar fashion, and the feud ended in September, when Nikki defeated Burchill on Superstars.
On October 12, the Bellas were traded back to Raw as part of a tri-branded Divas trade, where they predominately appeared in backstage segments with the weekly guest stars and only occasionally competed in matches. On the January 4, 2010, episode of Raw, Brie participated in a tournament for the vacant WWE Divas Championship, but lost to Maryse in the first round, when a switch resulted in Nikki being pinned. In June 2010, they developed a feud with Jillian Hall, when Brie defeated her after switching places with Nikki. The following week, Nikki defeated Hall after switching with Brie. The feud was exacerbated when the Bellas acted as the special guest referees during one of Hall's matches. During the match, Hall attacked both twins, but lost the match when Nikki made a fast count, allowing her to be pinned by Gail Kim. The next week on Superstars, the twins defeated Hall and Maryse in a tag team match to end the storyline.
On August 31, The Bella Twins announced they would be part of the all-female third season of NXT, mentoring Jamie. Jamie was the first rookie Diva eliminated on the October 5 episode of NXT. In November, the twins began a storyline with Daniel Bryan, when Brie accompanied him to the ring for his match. Following his win, Nikki ran out and the two fought over Bryan's affection, until Bryan broke it up and had them hug each other. They began to manage Bryan and frequently accompanied him to the ring over the next two months. In January 2011, both Bellas turned into villainous characters for the first time in their careers, when they discovered Bryan kissing Gail Kim backstage and assaulted her. They continued to attack Kim, both at the Royal Rumble and on Raw, and on February 7, they teamed with Melina in a losing effort to Kim, Eve Torres, and Tamina.
The Bellas began feuding with Eve Torres after they appeared as lumberjills during a Divas Championship match between Torres and Natalya on the February 14 episode of Raw. Following the match, they attacked Torres backstage before Gail Kim and Natalya stopped them. The next week, the twins defeated Torres and Kim in a tag team match. The following week, Nikki won a battle royal to become the number one contender for the Divas Championship, and unsuccessfully challenged Torres for the championship on March 7. On April 11, Nikki helped Brie to defeat Torres to win the Divas Championship, marking the first time either twin had held a championship in WWE. Nikki helped Brie once again, this time to successfully defend the championship against Kelly Kelly at Over the Limit, after switching places with Brie. However, Brie lost the championship against Kelly on a special "Power to the People" episode of Raw on June 20, ending her reign at 70 days, and failing to regain it on July 17, in a rematch against Kelly at Money in the Bank.
The twins spent the majority of the rest of the year in tag team matches, regularly facing Kelly and Torres. The Bellas began to show friction for the second time since joining WWE in March 2012, after both twins lost to AJ Lee in singles competition. After Brie's match with Lee, Nikki revealed that Brie was rooting for Team Johnny in the 12-man tag team match at WrestleMania XXVIII, whilst Nikki was rooting for Team Teddy, thus furthering their dissension.
On the April 6 episode of SmackDown, Nikki defeated then Divas Champion Beth Phoenix in a non-title match, after Kelly Kelly distracted Phoenix. On April 23, Nikki defeated Phoenix in a lumberjill match on Raw to win the Divas Championship for the first time. Brie lost Nikki's championship to Layla at Extreme Rules after Twin Magic failed, ending her Divas Championship reign after only a week. The following night on Raw, they competed in their last match with the WWE, failing to win back the Divas Championship from Layla in a triple threat match. Later that night, WWE announced on their website that the twins had been fired by Executive Administrator Eve Torres.
The Bella Twins returned to WWE on the March 11, 2013, episode of Raw in a backstage segment with Team Rhodes Scholars (Cody Rhodes and Damien Sandow). On the March 15 episode of SmackDown, the twins attacked The Funkadactyls (Cameron and Naomi), and the following week interfered in matches between Team Rhodes Scholars and Brodus Clay and Tensai, but were attacked by The Funkadactyls. The twins made their in–ring return facing and defeating The Funkadactyls on the March 27 episode of Main Event after interference from Cody Rhodes, and also defeated them on Raw five days later. The Bella Twins were scheduled to participate in an eight–person tag team match with Team Rhodes Scholars against Tons of Funk (Clay and Tensai) and The Funkadactyls at WrestleMania 29, but the match was cancelled due to time restraints, and instead took place the following night on Raw, where The Bella Twins and Team Rhodes Scholars were defeated. The twins continued their feud with The Funkadactyls throughout April, defeating them in standard tag team and six-Diva tag team matches. In June, Nikki suffered a fractured tibia.
Upon the debut of the Total Divas reality television program in July, The Bellas returned and began feuding with their co-star on the show, Natalya. Brie and Natalya went on to trade victories in singles competition on Raw and at SummerSlam. The cast of Total Divas then transitioned into a scripted feud with Divas Champion AJ Lee, who mocked the show and cast, turning The Bellas faces in the process. Nikki returned to in-ring action on the October 25 episode of SmackDown, losing to Lee. At Survivor Series, the following month, the twins were part of the victorious Team Total Divas. On the December 9 episode of Raw, the twins were awarded Slammy Awards for Diva of the Year.
On April 6, 2014, Nikki competed at WrestleMania XXX in the 14-Diva "Vickie Guerrero Invitational match" for the Divas Championship, which was won by the defending champion AJ Lee. The same month, Brie became involved in her real-life husband Daniel Bryan's ongoing storyline with Stephanie McMahon and Kane, where as part of the storyline, McMahon threatened to fire Brie if, an injured, Bryan did not relinquish the WWE World Heavyweight Championship at Payback, which forced Brie to "quit" WWE before slapping McMahon in the face. After Brie quit, McMahon put Nikki in several handicap matches as punishment. After a month absence, Brie returned to WWE television, appearing in the crowd on July 21; following a confrontation, McMahon slapped Brie and was subsequently arrested. In order to have Brie drop the "charges", Brie was rehired and received a match against McMahon at SummerSlam.
At the pay-per-view, Nikki turned heel by attacking Brie, which allowed McMahon to win the match. The next several weeks saw the twins fight in several backstage and in-ring segments, including a cameo appearance from Jerry Springer on Raw on September 8. As part of the storyline, McMahon declared Nikki the face of the Divas division and granted her a match at Night of Champions for the Divas Championship, which she failed to win. Nikki then obtained permission to begin forcing Brie to compete in handicap matches, similar to her punishment at the hands of McMahon, although Brie was able to win them. This led to a match between the twins at Hell in a Cell, where the loser was forced to become the winner's personal assistant for 30 days, in which Nikki defeated Brie. Five days later on SmackDown, Nikki won a Halloween costume battle royal to become the number one contender for the Divas Championship.
Nikki received her title match against AJ Lee on November 23 at Survivor Series, which she won with Brie's help to become a two-time Divas Champion. The duo had reconciled at this point, with Brie also turning heel in the process. Nikki then went on to retain her championship in three separate occasions – against Lee in a rematch on December 14, at TLC: Tables, Ladders & Chairs, against Naomi two days later on SmackDown, and against Paige at Fastlane on February 22, 2015. Paige and Lee then formed an alliance against Bellas, which led to a tag team match at WrestleMania 31, where Lee and Paige were victorious.
On April 26, after Nikki, with the help of Brie, once again retained her title against Naomi at Extreme Rules, a feud started, with Naomi aligning with the returning Tamina to even the odds against Bellas, who began showing more heroic characteristics. This led to a tag team match between the two teams on May 17 at Payback, where Naomi and Tamina were victorious. Two weeks later at Elimination Chamber, Nikki retained her title against Naomi and Paige in a triple threat match, with Brie banned from ringside.
In June, The Bella Twins became villains once again by employing Twin Magic, which helped Nikki retain the title against Paige on the June 1 episode of Raw and at Money in the Bank. During the feud with Paige, Alicia Fox allied with them to form Team Bella. At The Beast in the East on July 4, Nikki retained the title against Paige and Tamina. After weeks of Team Bella outnumbering Paige, Naomi, and Tamina, Stephanie McMahon called for a "revolution" in the WWE Divas division and introduced the debuting Charlotte and Becky Lynch as Paige's allies, while NXT Women's Champion Sasha Banks debuted as an ally to Naomi and Tamina, which led to a brawl between the three teams. Nikki then lost to Charlotte in a tag team match on the August 3 episode of Raw, and to Banks on the August 17 episode of Raw in a non-title match. The three teams faced off on August 23 at SummerSlam in a three team elimination match, in which Team Bella first eliminated Team B.A.D., before Team PCB's win.
On the September 14 episode of Raw, Nikki defended her title against Charlotte, who pinned Brie after the twins had switched places to win the match; however, since the title cannot change hands by disqualification, Nikki retained the championship, and in the process, became the new longest reigning Divas Champion in history, surpassing AJ Lee's previous record of 295 days. Nikki dropped the championship to Charlotte on September 20 at Night of Champions, ending her reign at 301 days, and failed to regain the title in a rematch on October 25 at Hell in a Cell.
Shortly after, Nikki went on a hiatus from television due to a neck injury which would require surgery, but returned for one night on December 21, to accept the Slammy Award for Diva of the Year. Nikki made a brief cameo appearance at the WrestleMania 32 event on April 3, 2016, celebrating with Brie after winning a 10-Diva tag team match. On July 21, she was revealed to be cleared for in-ring competition and continued her training at the WWE Performance Center.
After ten months of inactivity, Bella made a surprise return as a villainess at the SummerSlam pay–per–view on August 21, 2016, where she replaced Eva Marie and teamed with Natalya and Alexa Bliss against Becky Lynch, Naomi, and Carmella, whom they defeated after Bella pinned Carmella. This sparked a storyline feud between Bella (who was established as part of the SmackDown brand) and Carmella, also re-establishing Bella as a face in the process. Simultaneously with her feud with Carmella, Bella took part in a six–pack elimination challenge to determine the inaugural WWE SmackDown Women's Champion on September 11 at Backlash, where she eliminated Natalya but was eliminated by Carmella. Bella eventually gained revenge as she defeated Carmella on October 9 at No Mercy and also on December 4 at TLC: Tables, Ladders & Chairs in a no disqualification match, to end their feud.
In November, after she defeated Natalya, Bella became the team captain of Team SmackDown for the traditional five–on–five elimination match at Survivor Series. However, at the event, she was replaced by Natalya, after someone attacked her backstage before the match. In December, just after their feud ended, Carmella revealed Natalya as the attacker, who eventually confirmed it with the main reason being Bella's success and Natalya's downfall in the company. Upon the beginning of 2017, both women kept attacking each other inside the ring and backstage, which led to a match at the Elimination Chamber that ended in a double count–out. In a rematch that took place on the February 21 episode of SmackDown, Natalya defeated Bella in a falls count anywhere match after Maryse attacked Bella with a lead pipe because Bella accidentally tripped over her in the backstage make up area during her match with Natalya.
Maryse's attack sparked a feud between the two, which actually involved Bella's real real–life boyfriend John Cena and Maryse's real–life huseband, The Miz. This led to a mixed tag team match between the two couples at WrestleMania 33, in which Cena and Bella were victorious. After the match, Cena legitimately proposed to Bella, which she accepted. On April 4, Bella announced that she would be taking time off from wrestling.
The Bella Twins made an appearance at the 25th Anniversary of Raw on January 22, 2018. A few days later, both Nikki and Brie competed in the inaugural women's Royal Rumble match at the 2018 Royal Rumble where Nikki entered at number 27, lasting 16:30, and was the last woman eliminated by the eventual winner Asuka. In August, both twins made an appearance at the SummerSlam pay–per–view, showing their support for Ronda Rousey in her match with Alexa Bliss.
In September, The Bella Twins returned full-time on Raw and started feuding with The Riott Squad (Ruby Riott, Liv Morgan and Sarah Logan) over which they scored victories in various tag team and singles matches. To even the odds, Ronda Rousey aligned with The Bella Twins, and the trio was able to defeat The Riott Squad at WWE Super Show-Down in a six–woman tag team match. Two days later on the October 8 episode of Raw, after defeating The Riott Squad in a rematch, Nikki attacked Rousey, with Brie later following suit, turning villainous for the first time since 2015. Later that night, it was announced that Nikki would face Rousey for the Raw Women's Championship in the main event of the first ever all women's pay–per–view, WWE Evolution, in which she was unsuccessful despite multiple interferences by Brie. On June 20, 2019, Bella announced her official retirement.
On the February 21, 2020 edition of SmackDown, it was announced that Nikki and her twin sister Brie would be inducted into WWE Hall of Fame, during the A Moment of Bliss segment.