2021 Open de Oeiras III

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.

Seeds

 * 1 Rankings are as of 10 May 2021.

Other entrants
The following players received wildcards into the singles main draw:
 * Nuno Borges
 * João Domingues
 * Gastão Elias

The following player received entry into the singles main draw as an alternate:
 * Frederico Ferreira Silva

The following players received entry from the qualifying draw:
 * 🇧🇪 Zizou Bergs
 * 🇪🇨 Emilio Gómez
 * 🇫🇷 Hugo Grenier
 * 🇸🇰 Alex Molčan

Singles

 * 🇪🇸 Carlos Alcaraz def. 🇦🇷 Facundo Bagnis 6–4, 6–4.

Doubles

 * 🇺🇸 Hunter Reese / Sem Verbeek def. 🇫🇷 Sadio Doumbia / 🇫🇷 Fabien Reboul 4–6, 6–4, [10–7].