Selected in the fourth round of the 2018 NPB Draft from Meioh High School in Akita Prefecture,[1] he plays outfield and first base for the Chiba Lotte Marines.

Yamaguchi started for Meioh High School from his second year as a two-way player, batting cleanup as well as pitching.

He started as the designated hitter for the game and batted 5th, recording his first career hit in the 5th inning vs Hawks starter Shuta Ishikawa.

[4] Yamaguchi appeared in 78 games in total his rookie season, posting a slash line of .207/.365/.294 with 9 home runs.

[5] In the 6th inning of Game 2 of the 2021 Pacific League Climax Series First Stage, Yamaguchi hit a solo homer off Rakuten Golden Eagles's Tomohiro Anraku, becoming the youngest player to hit a home run in a Pacific League Climax Series game at the age of 21 years and 2 months.