| 2021 Open de Oeiras IV | |
|---|---|
| Date | 24–30 May |
| Edition | 4th |
| Surface | Clay |
| Location | Oeiras, Portugal |
| Champions | |
| Singles | |
 Gastão Elias | |
| Doubles | |
 Jesper de Jong / Tim van Rijthoven | |
The 2021 Open de Oeiras IV was a professional tennis tournament played on clay courts. It was the fourth edition of the tournament which was part of the 2021 ATP Challenger Tour. It took place in Oeiras, Portugal between 24 and 30 May 2021.
| Country | Player | Rank1 | Seed |
|---|---|---|---|
 FRA | Mathias Bourgue | 195 | 1 |
 TPE | Wu Tung-lin | 267 | 2 |
 GER | Julian Lenz | 269 | 3 |
| TPE | Tseng Chun-hsin | 274 | 4 |
 ECU | Roberto Quiroz | 275 | 5 |
 ESP | Carlos Gimeno Valero | 286 | 6 |
 ARG | Pedro Cachin | 292 | 7 |
 ITA | Riccardo Bonadio | 293 | 8 |
The following players received wildcards into the singles main draw:
Pedro Araújo Tiago Cação Luís FariaThe following player received entry into the singles main draw using a protected ranking:
The following player received entry into the singles main draw as an alternate:
The following players received entry from the qualifying draw:
Gastão Elias def. 
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