| 2021 Open de Rennes | |
|---|---|
| Date | 13–19 September |
| Edition | 15th |
| Surface | Hard (Indoor) |
| Location | Rennes, France |
| Champions | |
| Singles | |
 Benjamin Bonzi | |
| Doubles | |
 Bart Stevens / Tim van Rijthoven | |
The 2021 Open de Rennes was a professional tennis tournament played on hard courts. It was the fifteenth edition of the tournament and part of the 2021 ATP Challenger Tour. It took place in Rennes, France between 13 and 19 September 2021.
| Country | Player | Rank1 | Seed |
|---|---|---|---|
| FRA | Richard Gasquet | 79 | 1 |
| FRA | Arthur Rinderknech | 83 | 2 |
| FRA | Benjamin Bonzi | 94 | 3 |
| FRA | Gilles Simon | 103 | 4 |
 GBR | Andy Murray | 112 | 5 |
| FRA | Lucas Pouille | 133 | 6 |
| FRA | Grégoire Barrère | 136 | 7 |
 CZE | Tomáš Macháč | 143 | 8 |
The following players received wildcards into the singles main draw:
The following player received entry into the singles main draw as an alternate:
The following players received entry from the qualifying draw:
The following players received entry as lucky losers:
Benjamin Bonzi def. 
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