Kitamura began training in the New Japan Pro-Wrestling (NJPW) dojo in 2015. His first match in NJPW was not a traditional professional wrestling match, but rather a
dark amateur wrestling exhibition at
Lion's Gate Project 1, where he wrestled fellow trainee
Tomoyuki Oka to a time limit draw. After undergoing further training, Kitamura wrestled his first ever professional match on September 1, 2016, at
Lion's Gate Project 3, another dark match where he and Oka once again wrestled to a draw. On March 13, 2017, he made his televised debut, teaming with Oka in a loss to the
Guerrillas of Destiny (
Tama Tonga and
Tanga Loa). The day after that, he once again lost to Tonga and Loa, this time teaming with Nagata. On June 24, 2017, three months after his debut, Kitamura won the first match in his career when he teamed with Tomoyuki Oka to defeat
Manabu Nakanishi and Tetsuhiro Yagi in a tag match. Kitamura and Oka continued teaming regularly, eventually dubbing their team "Monster Rage". At the end of 2017, Kitamura took part in two tournaments held by NJPW; the
2017 World Tag League and the
2017 Young Lion Cup. In the World Tag League, Kitamura and fellow young lion
David Finlay finished last in their block after losing all seven of their matches. Meanwhile, in the Young Lion Cup, Kitamura emerged victorious after winning all five of his matches. On January 5 at New Year's Dash, Katsuya began a Seven Trial series of matches where he would face seven different wrestlers. He lost his first of seven matches to
Jay White at New Year Dash. At
the New Beginning in Sapporo he lost his second match of seven to
Michael Elgin and his third the following day to
Juice Robinson. At the Road to The New Beginning event from
Korakuen Hall he lost his fourth match against
Hiroyoshi Tenzan, and the fifth against
Manabu Nakanishi. At
The New Beginning in Osaka on February 10, he lost match #6 against
Yuji Nagata. At The New Beginning in Hiroshima he was scheduled to face Manabu Nakanishi, but he suffered a
cerebral concussion and was unable to compete. It was reported that the severity of the injury may force him to retire. He later injured his leg in a scooter accident. After almost a year with little news or updates on his condition, Kitamura left NJPW on January 31, 2019. == Mixed martial arts career ==