| Date: | 6–12 September |
| Edition: | 3rd |
| Surface: | Hard |
| Location: | Cassis, France |
| Champs: |  Benjamin Bonzi |
| Champd: |  Sriram Balaji / Ramkumar Ramanathan |
| Before Year: | 2019 |
The 2021 Cassis Open Provence was a professional tennis tournament played on hard courts. It was the 3rd edition of the tournament which was part of the 2021 ATP Challenger Tour. It took place in Cassis, France between 6 and 12 September 2021.
| Country | Player | Rank1 | Seed | |
|---|---|---|---|---|
| France | Benjamin Bonzi | 94 | 1 | |
 Japan | ||||
 Australia | ||||
 Kazakhstan | Mikhail Kukushkin | 129 | 4 | |
| France | Lucas Pouille | 133 | 5 | |
| France | Grégoire Barrère | 136 | 6 | |
 Austria | Jurij Rodionov | 140 | 7 | |
 United Kingdom | Liam Broady | 147 | 8 |
The following players received wildcards into the singles main draw:
The following players received entry into the singles main draw as alternates:
The following players received entry from the qualifying draw:
The following players received entry as lucky losers:
See main article: 2021 Cassis Open Provence – Singles.
Benjamin Bonzi def. Lucas Pouille 7–6(7–4), 6–4.See main article: 2021 Cassis Open Provence – Doubles.
Sriram Balaji / Ramkumar Ramanathan def. 
Hans Hach Verdugo / Miguel Ángel Reyes-Varela 6–4, 3–6, [10–6].