Though not under contract, Ruff made an appearance on the December 4, 2019 episode of WWE's NXT, teaming with Adrian Alanis in a losing effort against The Forgotten Sons (Steve Cutler and Wesley Blake). Throughout early 2020, Ruff wrestled as a local competitor on a variety of WWE television programs including Raw, SmackDown, NXT, 205 Live, and Main Event. On October 7, 2020, Ruff signed a contract with WWE and was assigned to the WWE Performance Center.
On the November 11 episode of NXT, Ruff defeated Johnny Gargano to win the NXT North American Championship. The following week on NXT, Ruff retained the championship against Gargano via disqualification when Damian Priest, Gargano's rival, intentionally punched Ruff. At NXT TakeOver: WarGames, Ruff lost the title back to Gargano in a triple threat match also involving Priest, ending Ruffs reign at twenty-five days. On the December 30 episode of NXT, Ruff faced Gargano in a rematch for the title but was defeated.
Leon Ruff would then begin teaming with Kushida and both men would enter the 2021 Dusty Rhodes Tag Team Classic. In the first round, Ruff and Kushida would defeat The Way (Johnny Gargano and Austin Theory) on the January 20 episode of NXT. On the January 27 episode of NXT, Ruff and Kushida would lose to The Grizzled Young Veterans in the quarterfinals. On the February 17 episode of NXT, Ruff would begin feuding with Isaiah "Swerve" Scott after Ruff defeated him in a match but was attacked by Scott afterwards. On the February 24 episode of NXT, Ruff was attacked by Scott before his match with Tyler Rust. On the March 31 episode of NXT, Ruff would qualify for the gauntlet eliminator on night one of NXT TakeOver: Stand and Deliver. At the event, Ruff was the first man eliminated in the match by Scott. On the April 13 episode of NXT, Ruff would be defeated by Scott in a one-on-one match and backstage after the match, Ruff would attack Scott. The two fought once more on the May 4 episode of NXT, with Ruff being defeated by Scott in a Falls Count Anywhere match due to interference from the debuting Top Dolla. On the May 11 episode of NXT, Ruff would be defeated by Pete Dunne despite attacking Dunne before the match had begun. On August 6, Ruff was released from his WWE contract. His final match took the place the same night with a win over Grayson Waller on 205 Live.