2023 Keio Challenger – Men's singles

Christopher O'Connell was the defending champion but chose not to defend his title.

Yosuke Watanuki won the title after defeating Yuta Shimizu 7–6(7–5), 6–4 in the final.

Seeds
• # 🇯🇵 Yosuke Watanuki (champion)

• # 🇦🇹 Jurij Rodionov (first round)

• # 🇺🇸 Michael Mmoh (semifinals)

• # 🇦🇺 James Duckworth (second round, withdrew)

• # 🇺🇸 Maxime Cressy (withdrew)

• # 🇮🇹 Luca Nardi (quarterfinals)

• # 🇦🇺 Marc Polmans (second round)

• # Leandro Riedi (second round)