Carlos Alcaraz defeated Richard Gasquet in the final, 6–2, 6–2 to win the singles tennis title at the 2021 Croatia Open. It was his first ATP Tour title. With the victory, Alcaraz became the youngest player to win an ATP Tour title since Kei Nishikori at the 2008 Delray Beach International Tennis Championships.[citation needed]
Dušan Lajović was the reigning champion from when the tournament was last held in 2019, but lost to Daniel Altmaier in the quarterfinals.
The top four seeds receive a bye into the second round.
Albert Ramos Viñolas (semifinals)
Dušan Lajović (quarterfinals) Filip Krajinović (quarterfinals)
Richard Gasquet (final)
Aljaž Bedene (first round) Jaume Munar (first round) Carlos Alcaraz (champion)
Gianluca Mager (second round)
Daniel Altmaier (qualified) Nikola Milojević (qualifying competition) Blaž Rola (first round) Alessandro Giannessi (qualified)
Filip Horanský (qualified)
Renzo Olivo (qualifying competition, lucky loser) Thomas Fabbiano (first round) Daniel Masur (first round)