He is also well known for his tenure in American promotion WWE where he worked under the name Hideo Itami (ヒデオ・イタミ) in the company's developmental territory NXT and later in their cruiserweight exclusive brand 205 Live.

In 2019, he departed from WWE, and in May, appeared for New Japan Pro-Wrestling (NJPW), announcing his participation in that year's G1 Climax and aligned with former tag partner Katsuyori Shibata.

He advanced to the finals by defeating Kotaro Suzuki and Tsuyoshi Kikuchi before losing to Yoshinobu Kanemaru on the Navigation with Breeze Tour in June 2002.

[15] Kenta's first major rivalry in singles competition began in March 2005 following a dispute with SUWA, which led to a series of confrontations between the two, including a violent match that ended in his opponent getting disqualified for assaulting the referee.

This saw Kenta shift his attention to the GHC Heavyweight Championship, challenging Naomichi Marufuji, losing in a match which won numerous awards and accolades from the wrestling media.

[23] Kenta lost the title in February 2009 to rival in their interpromotional rivalry with another member of the GPWA in the Kensuke Office, Katsuhiko Nakajima, only to win it back in the next month.

[25] Kobayashi made his return from injury on June 6, 2010, losing to Naomichi Marufuji in a GHC Junior Heavyweight Championship number one contender's match.

[26] On October 30, 2010, Kenta and Atsushi Aoki defeated Roderick Strong and Eddie Edwards in the finals of a tournament to win the 2010 Nippon TV Cup Junior Heavyweight Tag League.

[27] As a result, Kenta and Aoki received a shot at the GHC Junior Heavyweight Tag Team Championship, but were defeated by the champions, New Japan Pro-Wrestling representatives Koji Kanemoto and Tiger Mask, on November 23, 2010.

[28] On January 29, 2011, Kenta turned heel and join the Disobey stable with members Muhammad Yone, Yoshinobu Kanemaru and Genba Hirayanagi.

On May 25 Kenta and Yoshinobu Kanemaru defeated Atsushi Aoki and Kotaro Suzuki of the ANMU stable to win the vacant GHC Junior Heavyweight Tag Team Championship.

In November, Kenta entered Noah's Global Tag League and developed a new submission move he called "Game Over" to help him dispatch heavyweights.

On October 8, Kenta and Maybach Taniguchi defeated Magnus and Samoa Joe to win the GHC Tag Team Championship for the first time.

[36] On April 28, Kenta and Yoshihiro Takayama defeated Katsuhiko Nakajima and Kensuke Sasaki in the finals to win the 2013 Global Tag League.

[39] On June 2, Kenta defeated New Japan Pro-Wrestling representative and reigning GHC Tag Team Champion Toru Yano for his third successful title defense.

[47] On December 7, Kenta defeated New Japan representative and winner of the 2013 Global League, Yuji Nagata, for his ninth successful title defense.

His first appearance in the country was with Ring of Honor at Final Battle 2005, as a fan favorite, defending the GHC Junior Heavyweight title against Low Ki.

[64] Ring of Honor made its pay per view debut with the show, Respect is Earned, teaming with Nigel McGuinness against Danielson and Takeshi Morishima in a losing effort, after which Kenta spent several months with the company competing against rival, Davey Richards and his contingency, the No Remorse Corps.

Kenta made his next appearance on the Glory By Honor VI tour; the first night had him team with the GHC Heavyweight Champion Mitsuharu Misawa against Takeshi Morishima and Naomichi Marufuji, with the match ending in a thirty-minute time limit draw.

He returned a few weeks later to challenge Davey Richards once more, as well as make his second pay per view appearance for Take No Prisoners teaming with Tyler Black against Katsuhiko Nakajima and Austin Aries.

During the following month, Itami would have a feud with fellow Japanese wrestler Akira Tozawa, Mustafa Ali and was involved into the orbit for Cedric Alexander's Cruiserweight Championship.

[107] Kenta made his surprise debut for New Japan Pro-Wrestling (NJPW) on June 9, 2019, at the Dominion 6.9 in Osaka-jo Hall event as a face, with Katsuyori Shibata as his manager, announcing his participation in the 2019 G1 Climax tournament.

[117][118] On the final night of the tournament, Kenta competed in a six-man tag team match alongside Chaos members Tomohiro Ishii and Yoshi-Hashi against Bullet Club's Bad Luck Fale, Tama Tonga, and Tanga Loa, only to turn on them in the closing moments of the match and assisting the Bullet Club for the victory, turning heel.

[122] At Destruction in Kagoshima on September 16, Kenta returned and unsuccessfully faced Kota Ibushi for the Tokyo Dome IWGP Heavyweight Championship challenge rights certificate.

[126] In August, Kenta competed in the inaugural New Japan Cup USA tournament, with the winner receiving a match for the IWGP United States Heavyweight Championship.

[130] At Power Struggle on November 7, Kenta successfully retained his Right to Challenge contract for the United States Championship against Hiroshi Tanahashi.

[136] In March, Kenta competed in the New Japan Cup, defeating Juice Robinson and Minoru Suzuki, but losing to Shingo Takagi in the quarter-finals.

[137] In July at Wrestle Grand Slam in Tokyo Dome, Kenta competed in the New Japan Ranbo for the provisional KOPW Championship, but the match was won by Bullet Club stablemate Chase Owens.

[152] Later that month, Kenta returned to the US to make his first successful Strong Openweight Championship defense at Multiverse United, defeating Minoru Suzuki.

[154] In May, at Wrestling Dontaku, Kenta lost the Strong Openweight Championship to former Bullet Club stablemate, Hikuleo, ending his reign at 74 days.