Kosei Tani was born on 22 November 2000 in Sakai, Osaka, and rose through Gamba Osaka's youth setup. A 190cm goalkeeper with a calm demeanour and strong distribution, he was loaned to Shonan Bellmare from 2020 to 2022, where he established himself as a J1-level number one, playing 31 league matches in the 2022 season.
After a return to Gamba in 2023 and a brief loan to Belgian side Dender, Tani was loaned to Machida Zelvia in January 2024 — a move that became permanent in January 2025. With Machida he has been the everyday starter for two seasons running: 37 J1 League matches in 2024 and another 37 in 2025, plus 12 AFC Champions League Elite appearances during Machida's first ACL Elite campaign.
For Japan, Tani won the EAFF E-1 Football Championship in 2022 with the senior side and continues to feature in Japan goalkeeping camps as one of the strongest succession options to Zion Suzuki.
A modern, ball-playing goalkeeper with command of his box and confident distribution, Tani is one of the brightest J1 number ones in his age bracket. He is in the conversation for the 2026 FIFA World Cup squad as a senior backup goalkeeper, alongside Daniel Schmidt and Shuichi Gonda.