Koki Machida was born on 25 August 1997 in Ibaraki, Japan, and came through Kashima Antlers' youth ranks. With Kashima he won the J.League Cup in 2015, the J1 League in 2016, the Emperor's Cup in 2016–17, the Super Cup in 2017, and the AFC Champions League in 2018 — a remarkable haul for a young centre-back at one of Japan's most-decorated clubs.
In January 2022 Belgian side Royale Union Saint-Gilloise signed Machida on loan, with a permanent move in summer 2023. Across his Union years he won the Belgian Cup in 2023–24, the Belgian Super Cup in 2024, and most notably the Belgian First Division A in 2024–25 — Union's first league title in nearly a century. He played 34 league matches with 1 goal in the 2024–25 title-winning season, plus 8 UEFA Europa League and 2 UEFA Champions League appearances.
In July 2025 Bundesliga side 1899 Hoffenheim signed Machida. His Bundesliga debut has been limited so far: 1 league match and a DFB-Pokal appearance.
A 190cm two-footed centre-back with elite aerial dominance, ball-playing security and the senior leadership of a player who has won multiple major Asian and European honours, Machida has been a Japan international since 2023, including the 2023 AFC Asian Cup. He is a senior centre-back option for the 2026 FIFA World Cup as a competitor for Itakura, Seko and Takai's CB roles.