Ryuta Koike was born on 29 August 1995 in Hachioji, Tokyo. He came through Renofa Yamaguchi's youth football before joining Kashiwa Reysol in January 2017. After a loan to Belgian side Lokeren in summer 2019, he moved to Yokohama F. Marinos in summer 2020.
With F. Marinos he won the J1 League in 2022, the Super Cup in 2023, and the EAFF E-1 Football Championship with Japan in 2022 — Marinos' senior right-back during their most-decorated period.
In January 2025 he moved to Kashima Antlers. The 2025 season has been productive: 29 J1 League matches with a goal and 3 assists, plus 2 J.League Cup appearances.
A 170cm two-footed right-back with elite acceleration, attacking instincts and the work rate to defend, Koike has been a Japan call-up across multiple senior camps. He is a candidate for the wider 2026 FIFA World Cup squad as a right-back depth option.