See main article: 2021 Open de Oeiras III.
Nuno Borges and Francisco Cabral were the defending champions[1] but lost in the semifinals to Hunter Reese and Sem Verbeek.
Reese and Verbeek won the title after defeating Sadio Doumbia and Fabien Reboul 4–6, 6–4, [10–7] in the final.