- Source: 2021 Open de Oeiras IV
The 2021 Open de Oeiras IV was a professional tennis tournament played on clay courts. It was the fourth edition of the tournament which was part of the 2021 ATP Challenger Tour. It took place in Oeiras, Portugal between 24 and 30 May 2021.
Singles main-draw entrants
= Seeds
=1 Rankings are as of 17 May 2021.
= Other entrants
=The following players received wildcards into the singles main draw:
Pedro Araújo
Tiago Cação
Luís Faria
The following player received entry into the singles main draw using a protected ranking:
Viktor Galović
The following player received entry into the singles main draw as an alternate:
Corentin Denolly
The following players received entry from the qualifying draw:
Filip Cristian Jianu
Nikolás Sánchez Izquierdo
Timofey Skatov
Denis Yevseyev
Champions
= Singles
=Gastão Elias def. Holger Rune 5–7, 6–4, 6–4.
= Doubles
=Jesper de Jong / Tim van Rijthoven def. Julian Lenz / Roberto Quiroz 6–1, 7–6(7–3).