| Champ: |  Jannik Sinner |
| Runner: |  Mackenzie McDonald |
| Score: | 7–5, 4–6, 7–5 |
| Draw: | 48 (6 / 4) |
| Seeds: | 16 |
| Type: | singles doubles |
| Before Year: | 2019 |
| Before Name: | Citi Open – Men's singles |
| Main Name: | Washington Open (tennis) Washington Open |
| After Name: | Citi Open – Men's singles |
See main article: article and 2021 Citi Open.
Jannik Sinner defeated Mackenzie McDonald in the final, 7–5, 4–6, 7–5 to win the singles tennis title at the 2021 Citi Open. With the victory, Sinner became the youngest player and first teenager on the ATP Tour to win an ATP 500 title since the category was created in 2009.
Nick Kyrgios was the reigning champion from when the tournament was last held in 2019,[1] but lost in the first round to McDonald.
All seeds receive a bye into the second round.