2021 Kremlin Cup – Men's singles

Andrey Rublev was the reigning champion from when the tournament was last held in 2019,[1] but lost to Adrian Mannarino in the second round in a rematch of the 2019 final.

Aslan Karatsev won the title, defeating Marin Čilić in the final, 6–2, 6–4.

The top four seeds received a bye into the second round.