| Champ: | Marin Čilić |
| Runner: |  Taylor Fritz |
| Score: | 7–6(7–3), 4–6, 6–4 |
| Draw: | 28 (4 / 3) |
| Seeds: | 8 |
| Type: | singles doubles |
| Before Name: | St. Petersburg Open – Singles |
| After Name: | St. Petersburg Open – Singles |
See main article: article and 2021 St. Petersburg Open.
Andrey Rublev was the defending champion,[1] but lost in the quarterfinals to Botic van de Zandschulp.
Marin Čilić won the title, defeating Taylor Fritz in the final, 7–6(7–3), 4–6, 6–4.
The top four seeds received a bye into the second round.