The 2021 Estoril Open is a men's tennis tournament played on outdoor clay courts.

It is the 6th edition of the tournament and part of the ATP Tour 250 series of the 2021 ATP Tour.

It tames place at the Clube de Ténis do Estoril in Cascais, Portugal, from 26 April through 2 May 2021.

Seventh-seeded Albert Ramos Viñolas won the singles title.

[1] *per team The following players received wildcards into the main draw: The following players received entry from the qualifying draw: The following player received entry as a lucky loser: The following pairs received wildcards into the doubles main draw: ]