Koki Anzai was born on 31 May 1995 in Kawaguchi, Saitama. He came through Tokyo Verdy's youth ranks before joining Kashima Antlers in January 2018. With Kashima he won the AFC Champions League in 2018 — the club's third continental crown.
After a loan to Portuguese side Portimonense in summer 2019, he returned to Kashima permanently in July 2021. The 2023 Kashima season produced 31 J1 matches with 4 assists, 2024 brought 38 J1 matches with a goal and an assist plus a J.League Cup goal, and 2025 produced 19 J1 matches with 4 assists across the first half of the season.
A 172cm two-footed left-back with elite acceleration, attacking instincts and the work rate to defend, Anzai has been a Japan call-up across multiple senior camps. He is one of Kashima's most senior left-sided defenders and is a candidate for the wider 2026 FIFA World Cup squad as a left-back depth option.