| Date: | 24–30 May |
| Edition: | 4th |
| Surface: | Clay |
| Location: | Oeiras, Portugal |
| Champs: | Gastão Elias |
| Champd: | Jesper de Jong / Tim van Rijthoven |
| Before Name: | Open de Oeiras III |
| Before Year: | 2021 |
| Main Name: | Open de Oeiras |
| After Name: | Open de Oeiras |
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 | |
|---|---|---|---|---|
 France | Mathias Bourgue | 195 | 1 | |
| Wu Tung-lin | 267 | 2 | ||
| Julian Lenz | 269 | 3 | ||
| Tseng Chun-hsin | 274 | 4 | ||
 Ecuador | Roberto Quiroz | 275 | 5 | |
 Spain | Carlos Gimeno Valero | 286 | 6 | |
 Argentina | Pedro Cachin | 292 | 7 | |
 Italy | Riccardo Bonadio | 293 | 8 |
The following players received wildcards into the singles main draw:
The 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:
See main article: 2021 Open de Oeiras IV – Singles.
See main article: 2021 Open de Oeiras IV – Doubles.
Roberto Quiroz 6–1, 7–6(7–3).
