Tomoki Iwata was born on 7 April 1997 in Usa, Oita. He came through Oita Trinita's youth ranks and joined Yokohama F. Marinos in January 2021. With F. Marinos he won the J1 League in 2022 and the EAFF E-1 Football Championship with Japan in 2022.
In January 2023 Celtic signed Iwata. With Celtic he won the Scottish Premiership and Scottish League Cup in 2022–23, plus the Premiership and Scottish Cup in 2023–24 — back-to-back Premiership titles. He played 19 Premiership matches with a goal and an assist in 2023.
In August 2024 Birmingham City signed Iwata. His League One debut season was outstanding: 40 league matches with 6 goals and an assist, 2 FA Cup matches with a goal, and 5 EFL Trophy matches with a goal as Birmingham earned the League One title in 2024–25 and promotion. The 2025–26 Championship season has continued his form: 45 league matches with 3 goals and an assist.
A 178cm two-footed central midfielder/centre-back with versatility, ball-playing security and the goal-scoring instinct from arriving in the box, Iwata is one of Japan's most decorated central midfielders. He is a candidate for the wider 2026 FIFA World Cup squad as a senior versatile option.