David Goffin was the defending champion, but lost in the second round to Andy Murray.
Gong Maoxin and Zhang Ze were the defending champions but chose not to defend their title.
Marcelo Demoliner and David Vega Hernández were the defending champions but only Vega Hernández chose to defend his title, partnering Simone Bolelli. Vega Hernández successfully defended his title.
Purav Raja and Antonio Šančić were the defending champions but only Raja chose to defend his title, partnering Rameez Junaid. Raja lost in the first round to Scott Clayton and Evan Hoyt.
Tomislav Brkić and Ante Pavić were the defending champions but chose not to defend their title.
Kevin Krawietz and Andreas Mies were the defending champions but chose not to defend their title.
This was the first edition of the tournament and first of two editions of the tournament to start the 2021 ATP Challenger Tour year.
Nikoloz Basilashvili defeated Roberto Bautista Agut in the final, 7–6(7–5), 6–2 to win the men's singles tennis title at the 2021 Qatar Open. Basilashvili saved a match point against Roger Federer in the quarterfinals.
Evan King and Julian Lenz were the defending champions but only King chose to defend his title, partnering Jan Zieliński. King lost in the first round to Franco Agamenone and Omar Giacalone.
Denis Kudla was the defending champion but chose not to defend his title.
Albano Olivetti and David Vega Hernández were the defending champions but only Olivetti chose to defend his title, partnering Hunter Reese. Olivetti lost in the quarterfinals to Anirudh Chandrasekar and Arjun Kadhe.
Purav Raja and Ramkumar Ramanathan were the defending champions but only Raja chose to defend his title, partnering Petr Nouza. Raja lost in the quarterfinals to Anirudh Chandrasekar and Vijay Sundar Prashanth.
Marcelo Demoliner and Jan-Lennard Struff were the defending champions but chose not to defend their title.
Yuki Bhambri and Saketh Myneni were the defending champions but chose not to defend their title.
Dustin Brown and Andrea Vavassori were the defending champions but chose not to defend their title.
Nam Ji-sung and Song Min-kyu were the defending champions but only Nam chose to defend his title, partnering Hans Hach Verdugo. Nam lost in the quarterfinals to Francis Alcantara and Kaichi Uchida.
Anirudh Chandrasekar and Vijay Sundar Prashanth were the defending champions but chose to defend their title with different partners. Chandrasekar partnered Arjun Kadhe but lost in the first round to Marco Bortolotti and Alexandru Jecan. Prashanth partnered Jeevan Nedunchezhiyan but lost in the quarterfinals to Vladyslav Manafov and Patrik Niklas-Salminen.
Alex Bolt and Andrew Harris were the defending champions but chose not to defend their title.
Toshihide Matsui and Kaito Uesugi were the defending champions but lost in the quarterfinals to Arjun Kadhe and Arthur Reymond.
Théo Arribagé and Luca Sanchez were the defending champions but only Sanchez chose to defend his title, partnering Divij Sharan. They lost in the first round to Michael Geerts and Szymon Walków.
David Pichler 
Anirudh Chandrasekar 
David Vega Hernández 
