Kotaro Hayashi

Hayashi started his youth football career with Sagan Tosu U-15s and quickly made his way up to the U-18s, being named as a substitute for them in 2016 at just 15. He made his debut for the U-18s in 2017 and became a regular starter in the 2018 season, captaining Sagan Tosu U-18s and helping them finish top of Prince Takamado JFA U-18 Prince League Kyushu and reach the quarter-finals of the Japan Club Youth Cup. In February 2019, Meiji University announced that Hayashi would be joining their team. He became a registered member of the team in 2021. He made his debut for Meiji University in April 2021, coming off the bench to replace Ren Kato in a 2–1 JUFA Kanto League 1 match against the University of Tsukuba. He made 15 appearances in his first season, scoring two goals. In his final year, Hayashi was named captain and helped Meiji University win the league, despite fracturing his metatarsal midway through the season.

Yokohama FC

Hayashi was offered a professional contract with Yokohama FC after playing a training game against them for Meiji University – a match he was not originally scheduled to take part in but played due to a clash with exams. It was officially announced in July 2022. He made his debut for the club in March 2023 in a 3–1 league defeat to Kashima Antlers. He also scored his first professional goal in his debut season, scoring in a 1–1 league draw with Sanfrecce Hiroshima. He made 32 appearances and scored two goals across all competitions in his debut season, but was unable to prevent Yokohama FC from being relegated to the J2 League.

Machida Zelvia

In January 2024, it was announced that Hayashi would be transferring to newly promoted J1 League club Machida Zelvia ahead of the 2024 season. He made 37 appearances across all competitions throughout the season, helping Machida to a third-placed finish. In the 2025 Emperor's Cup, Hayashi scored the opening goal in the semi-final as Machida beat FC Tokyo 2–0 after extra time. The club went on to win the competition, defeating Vissel Kobe in the final – Hayashi's first piece of silverware. Machida Zelvia's qualification for the AFC Champions League gave Hayashi his continental debut, in a 1–1 draw with FC Seoul during the 2025–26 season. He went on to make 13 appearances in the competition as Machida reached the final. Hayashi started the final, but Machida were beaten 1–0 after extra time by Al-Ahli.

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Hayashi is a right-footed left back and has named one of his favourite foreign players as João Cancelo. Due to the tactics played by Machida Zelvia during the 2024 season, Hayashi has been effective at long throw-ins. Hayashi is a hard-working player who is adept in one-on-one situations.

Hayashi was born in Ureshino, Saga and started playing football in the third grade of elementary school. He studied at Saga Prefectural Konan Junior High School and later Saga Prefectural Sagakita High School.