| Date: | 29 March – 3 April |
| Edition: | 1st |
| Surface: | Clay |
| Location: | Oeiras, Portugal |
| Champs: |  Zdeněk Kolář |
| Champd: | Mats Moraing / Oscar Otte |
| After Name: | Open de Oeiras II |
| After Year: | 2021 |
The 2021 Open de Oeiras was a professional tennis tournament played on clay courts. It was the first edition of the tournament which was part of the 2021 ATP Challenger Tour. It took place in Oeiras, Portugal between 29 March and 3 April 2021.
| Country | Player | Rank1 | Seed | |
|---|---|---|---|---|
| Oscar Otte | 154 | 1 | ||
 France | Enzo Couacaud | 185 | 2 | |
 India | ||||
| Ernesto Escobedo | 205 | 4 | ||
 Argentina | Andrea Collarini | 215 | 5 | |
 Morocco | Elliot Benchetrit | 240 | 6 | |
 Italy | Gian Marco Moroni | 241 | 7 | |
| Czech Republic | Zdeněk Kolář | 246 | 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:
See main article: 2021 Open de Oeiras – Singles.
Zdeněk Kolář def. Gastão Elias 6–4, 7–5.See main article: 2021 Open de Oeiras – Doubles.
Riccardo Bonadio / 
Denis Yevseyev 6–1, 6–4.