Ayumu Seko was born on 7 June 2000 in Osaka's Taisho Ward and joined Cerezo Osaka's academy at the age of 10, climbing through every age group at the club. He made his senior debut as a teenager and was on the bench when Cerezo lifted the J.League Cup in 2017. By 2020 he was a regular J1 League starter at centre-back, despite being one of the youngest players in the league.
In January 2022 he made his European move to Switzerland with Grasshopper Club Zürich. Three full seasons in the Super League — including 37-match starting roles in 2023–24 and 2024–25 — established him as a calm, technically solid centre-back who could defend in transition and play out from the back. In July 2025 he stepped up to France's Ligue 1, joining Le Havre on a free transfer.
His debut Ligue 1 season has been outstanding: he has been an everyday starter, with 29 appearances and over 2,500 minutes by mid-season — defending in a structured, low-block system and maturing into a high-level centre-back. Tall (185cm), composed and two-footed, Seko is a centre-back with the profile to operate in any modern back line.
He has been a Japan A-team regular since 2023 and is in line to be one of Japan's first-choice centre-backs at the 2026 FIFA World Cup, alongside more senior names like Maya Yoshida (now retired from internationals) and Ko Itakura.