On July 31, 2018, it was reported that Riddle had signed a contract with WWE. On August 18, Riddle appeared at NXT TakeOver: Brooklyn IV and was identified by the event's commentators as WWE's newest signing.
Riddle's NXT debut was on the October 31 edition when he successfully defeated Luke Menzies. On November 17, 2018 on the pre-show of NXT TakeOver: WarGames II, Kassius Ohno interrupted a Riddle interview. When NXT TakeOver: WarGames II started, Riddle challenged Ohno to a match and knocked him out in 6 seconds, winning the fastest match in NXT history. Riddle and Ohno had a rematch at NXT TakeOver: Phoenix, where Riddle made Ohno tap out to an Arm Trap.
Throughout March, Riddle would start a feud with the NXT North American Champion Velveteen Dream. On April 5, at NXT TakeOver: New York, Riddle challenged Velveteen Dream for the NXT North American Championship. However, Dream would defeat Riddle ending his undefeated streak in NXT. In July, Riddle started a feud with the returning Killian Dain. On the August 7 episode of NXT, Riddle was supposed to face Dain but it never happened since they brawled before the match happened. The two brawled at NXT TakeOver: Toronto and eventually faced each other on the August 21 episode of NXT where Dain was victorious, although Riddle attacked Dain after the match. On the premiere episode of NXT on the USA Network on September 18, Riddle fought Dain in a Street Fight but it ended in a no contest. A week later on the September 25 episode of NXT, Riddle again fought Dain in a Street Fight but this time the winner would be the #1 Contender for the NXT Championship, which Riddle won by submission. On the October 2nd episode of NXT, Riddle unsuccessfully challenged NXT Champion Adam Cole.
On the November 1 episode of SmackDown, Riddle and Keith Lee were one of the many NXT wrestlers to invade the show, confronting and ultimately attacking Sami Zayn. Later that night, Riddle joined Triple H and the rest of the NXT roster as they declared war on both Raw and SmackDown, and vowed to win the Survivor Series brand warfare. In November, Riddle would start feuding with Finn Bálor. On the November 13 episode of NXT, Finn Bálor insulted the NXT roster as “all boys who can't take a beating”, specifically mentioning Johnny Gargano and Matt Riddle. Riddle attacked Bálor, who retreated. Riddle was originally part of Team Ciampa for NXT TakeOver: WarGames, but on the November 13 episode of NXT, Riddle was taken off Team Ciampa due to Riddle having a match with Bálor at WarGames. At the event, Riddle was defeated.
On the January 10, 2020 episode of NXT, the participants of the 2020 Dusty Rhodes Tag Team Classic were announced, with Riddle and Pete Dunne being announced as surprise participants; they would later take on the name of The Broserweights, a portmanteau of Riddle's "Original Bro" nickname and Dunne's "Bruiserweight" nickname. Together, they defeated Mark Andrews and Flash Morgan Webster in the first round on January 15, Fabian Aichner and Marcel Barthel of Imperium in the semifinals on January 22, and Grizzled Young Veterans (James Drake and Zack Gibson) in the finals on January 29 to win the tournament and earn a match against Bobby Fish and Kyle O'Reilly of The Undisputed Era for the NXT Tag Team Championship at NXT TakeOver: Portland. Riddle also participated in the Royal Rumble match at the namesake pay-per-view and entered at number twenty-three, but was eliminated by King Corbin. At NXT TakeOver: Portland on February 16, Riddle and Dunne defeated Fish and O'Reilly to win the titles, marking Riddle's first title win in WWE. When Dunne was out indefinitely, Timothy Thatcher substituted him. Riddle and Thatcher would lose the titles against Imperium (Fabian Aichner and Marcel Barthel) on May 13, 2020. On the May 27 episode of NXT, Riddle and Thatcher fought in a Fight Pit match where Riddle lost to Thatcher by technical submission. This would turn out to be Riddle's last match in NXT.
On May 29, 2020, Kurt Angle announced that Riddle would be moving to SmackDown. On the June 19 edition, he debuted by crashing AJ Styles' WWE Intercontinental Championship celebration. This led to a match, which Riddle won after interference from other superstars that were outside of the ring. On the July 3, 2020 episode of SmackDown, Riddle defeated John Morrison.
As part of the 2020 draft in October, Riddle was drafted to the Raw brand. On October 29, his ring name was shortened to Riddle. Riddle would participate on Team Raw in the men's traditional elimination tag team match at Survivor Series, where he would pin King Corbin on route to a clean sweep for Raw. In December, Riddle would begin a feud with United States Champion Bobby Lashley. On the January 4, 2021 episode of Raw, Riddle defeated Lashley in a non-title match and would earn a title match the following week, but failed to capture the championship. On the January 25 episode, Riddle defeated Cedric Alexander, MVP, and Shelton Benjamin in a gauntlet match to earn another title opportunity against Lashley. At the Royal Rumble, Riddle entered at number sixteen and helped eliminate Lashley before being eliminated by Seth Rollins. Riddle would receive his title match the following night on Raw, where he defeated Lashley by disqualification but did not win the championship.
On February 21, Riddle defeated Bobby Lashley and John Morrison in a triple threat match at the Elimination Chamber pay-per-view to win the WWE United States Championship, his first title on the main roster and his first singles title in WWE. He lost the title to Sheamus on night two of WrestleMania 37, ending his reign at 49 days.
On the April 19 episode of Raw, Riddle defeated Randy Orton in a singles match, leading to Orton forming a tag team with Riddle called "RK-Bro". The team would proceed to defeat Cedric Alexander and Shelton Benjamin and Elias and Jaxson Ryker in the coming weeks. On the May 11 episode of Raw, after RK-Bro and The New Day beat Ryker, Elias, and Raw Tag Team Champions AJ Styles and Omos in an eight-man tag team match, Orton RKO'ed both Kofi Kingston and Xavier Woods. On the June 21 episode of Raw, Riddle would defeat Drew McIntyre to qualify for the Money in the Bank ladder match at Money in the Bank. The following week, Orton was scheduled to face AJ Styles and Drew McIntyre in a last chance match, but for unknown reasons was pulled and replaced by Riddle, who would go on to lose the match for Orton. After a seven week absence, Orton returned on the August 9 episode of Raw, where he initially discontinued his team with Riddle at the beginning of the show. Later on in the night, Orton defeated AJ Styles in a match following assistance from Riddle and afterwards pretended to hug him, but instead hit him with an RKO as a sign of gratitude. The following week, Orton officially reunited the team after Riddle saved Orton from an attack at the hands of Omos and Styles. At SummerSlam, RK-Bro defeated Styles and Omos to win their first WWE Raw Tag Team Championship, both individually and as a team. At Crown Jewel, RK-Bro would retain their titles against AJ Styles and Omos.
While performing on Raw, Riddle returned to NXT on the December 7 episode, when he was revealed to be the shaman for MSK. On the January 10, 2022 episode of Raw, RK-Bro lost the titles to Alpha Academy (Chad Gable and Otis), ending their reign at 142 days. Riddle then participated in the Royal Rumble match at the namesake event before being eliminated by the eventual winner Brock Lesnar. On the March 7 episode of Raw, RK-Bro won their second Raw Tag Team Championship after defeating Alpha Academy and the team of Kevin Owens and Seth Rollins in a triple threat tag team match. They then successfully defended the titles at WrestleMania 38, also in a triple threat tag team match, involving Alpha Academy and The Street Profits. Two weeks after WrestleMania 38, SmackDown Tag Team Champions The Usos appeared on Raw, challenging RK-Bro to a tag team title unification match at WrestleMania Backlash, with RK-Bro accepting the challenge. However, the match would soon be changed to six-man tag match with RK-Bro aligning with Drew McIntyre to take on The Usos and Roman Reigns. At the event, The Bloodline would be successful. On the May 20 episode of SmackDown, RK-Bro were defeated by the Usos, therefore losing the Raw Tag Team Championships and making the Usos the new undisputed tag champs. After Orton was sidelined with a back injury, Riddle continued to feud with The Bloodline, teaming up with common Bloodline enemy Shinsuke Nakamura to unsuccessfully challenge The Usos for the Undisputed WWE Tag Team Championship on the June 3 episode of SmackDown following a distraction from Bloodline associate Sami Zayn. With a win over Zayn the following week, Riddle earned a match against Reigns for the Undisputed WWE Universal Championship. On the June 17 episode of SmackDown, Riddle lost to Reigns, barring him from challenging Reigns for the title again.
On the June 20 episode of Raw, Riddle failed to qualify for the Money in the Bank ladder match after losing to Omos. However, he won a last chance qualifying battle royal the following week to earn his way into the match. At the eponymous event on July 2, he was unsuccessful in winning the match. He then began feuding with Seth Rollins, leading to a match between the two being announced for SummerSlam. However, the match was postponed by WWE after Riddle suffered an injury following an attack by Rollins on the July 25 episode of Raw. Despite the injury, Riddle appeared at SummerSlam and called out Rollins, leading to a brawl which saw Rollins come out on top. On the August 29 episode of Raw, his ring name was reverted back to Matt Riddle. Riddle and Rollins finally faced off at Clash at the Castle on September 3, where Riddle was defeated. On the September 20 episode of Raw, Riddle got involved in a United States Championship match between the champion Bobby Lashley and Rollins, causing Rollins to lose the match. Later in the show, during another brawl, Riddle challenged Rollins to a Fight Pit match for the Extreme Rules event. The match was confirmed by WWE, making it the first Fight Pit match on the main roster. At the event on October 8, Riddle defeated Rollins via submission. On the October 17 episode of Raw, Riddle failed to win the United States Championship from Rollins, ending their feud.
On December 13, it was reported that Riddle had been suspended for sixty days after failing WWE's wellness policy. On April 3, 2023, Riddle returned on the Raw after WrestleMania, attacking The Miz. On the April 17 episode of Raw, the team of Riddle, Kevin Owens and Sami Zayn defeated The Judgment Day's Finn Bálor, Damian Priest and Dominik Mysterio in a six-man tag team match. After the match, the trio was assisted by the Latino World Order in their brawl against The Judgment Day and The Bloodline. On September 22, Riddle was released from his WWE contract.