| Champ: |  Carlos Alcaraz |
| Runner: |  Richard Gasquet |
| Score: | 6–2, 6–2 |
| Draw: | 28 (4 / 3) |
| Seeds: | 8 |
| Type: | singles doubles |
| Before Year: | 2019 |
See main article: article and 2021 Croatia Open Umag.
Carlos Alcaraz defeated Richard Gasquet in the final, 6–2, 6–2 to win the singles tennis title at the 2021 Croatia Open. It was his first ATP Tour title. With the victory, Alcaraz became the youngest player to win an ATP Tour title since Kei Nishikori at the 2008 Delray Beach International Tennis Championships.
Dušan Lajović was the reigning champion from when the tournament was last held in 2019, but lost to Daniel Altmaier in the quarterfinals.
The top four seeds receive a bye into the second round.