| Date: | 17–23 May |
| Edition: | 3rd |
| Surface: | Clay |
| Location: | Oeiras, Portugal |
| Champs: |  Carlos Alcaraz |
| Champd: |  Hunter Reese / Sem Verbeek |
| Before Name: | Open de Oeiras II |
| Before Year: | 2021 |
| Main Name: | Open de Oeiras |
| After Name: | Open de Oeiras IV |
| After Year: | 2021 |
The 2021 Open de Oeiras III was a professional tennis tournament played on clay courts. It was the third edition of the tournament which was part of the 2021 ATP Challenger Tour. It took place in Oeiras, Portugal between 17 and 23 May 2021.
| Country | Player | Rank1 | Seed | |
|---|---|---|---|---|
 Czech Republic | Jiří Veselý | 73 | 1 | |
| Steve Johnson | 85 | 2 | ||
 Argentina | Federico Coria | 91 | 3 | |
| Spain | Pedro Martínez | 94 | 4 | |
| Yannick Hanfmann | 95 | 5 | ||
| Argentina | Juan Ignacio Londero | 98 | 6 | |
| Argentina | Facundo Bagnis | 105 | 7 | |
| Pedro Sousa | 111 | 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:
See main article: 2021 Open de Oeiras III – Singles.
Carlos Alcaraz def. Facundo Bagnis 6–4, 6–4.See main article: 2021 Open de Oeiras III – Doubles.
Hunter Reese / Sem Verbeek def. 
Sadio Doumbia / Fabien Reboul 4–6, 6–4, [10–7].
