| Champ: |  Rajeev Ram Joe Salisbury |
| Runner: | Nikola Mektić Mate Pavić |
| Score: | 6–3, 4–6, [10–3] |
| Before Name: | Rogers Cup – Men's doubles |
| Before Year: | 2019 |
See main article: 2021 National Bank Open.
Marcel Granollers and Horacio Zeballos were the defending champions from when the tournament was last held in 2019,[1] but withdrew before the tournament began.
Rajeev Ram and Joe Salisbury won the title, defeating Nikola Mektić and Mate Pavić in the final, 6–3, 4–6, [10–3]. By reaching the finals, the Croatian tandem became the second team since the Bryans in 2014, to reach 5 consecutive Masters 1000 finals.
The top four seeds receive a bye into the second round.