The 2021 Sofia Open was a men's tennis tournament played on indoor hard courts. It was the sixth edition of the Sofia Open as part of the ATP World Tour 250 series of the 2021 ATP Tour. It was played at the Arena Armeec in Sofia, Bulgaria, from 27 September to 3 October 2021. First-seeded Jannik Sinner won the singles title.
Finals
Singles
Jannik Sinner defeated Gaël Monfils 6–3, 6–4
Doubles
Jonny O'Mara / Ken Skupski defeated Oliver Marach / Philipp Oswald, 6–3, 6–4
Singles main-draw entrants
Seeds
1 Rankings are as of 20 September 2021.
Other entrants
The following players received wildcards into the main draw: Adrian Andreev Dimitar Kuzmanov Alexandar Lazarov
The following players received entry from the qualifying draw: Egor Gerasimov Illya Marchenko Pedro Martínez Andreas Seppi
The following player received entry as a lucky loser: Kamil Majchrzak
Withdrawals
Before the tournament
Roberto Bautista Agut → replaced by James Duckworth
Alexander Bublik → replaced by Kamil Majchrzak
Marin Čilić → replaced by Gianluca Mager
Mackenzie McDonald → replaced by Emil Ruusuvuori
During the tournament
Ilya Ivashka
Doubles main-draw entrants
Seeds
1 Rankings are as of 20 September 2021.
Other entrants
The following pairs received wildcards into the doubles main draw: Adrian Andreev / Alexandar Lazarov Alexander Donski / Dimitar Kuzmanov
The following pair received entry as alternates: Plamen Milushev / Radoslav Shandarov
Withdrawals
Before the tournament
Jérémy Chardy / Hugo Nys → replaced by André Göransson / Hugo Nys
Matthew Ebden / Jonathan Erlich → replaced by Jonathan Erlich / Dominic Inglot Sander Gillé / Joran Vliegen → replaced by Jonny O'Mara / Ken Skupski Marcos Giron / Mackenzie McDonald → replaced by Lorenzo Musetti / Andrea Vavassori Pedro Martínez / Jaume Munar → replaced by Plamen Milushev / Radoslav Shandarov