| Date: | 18–24 October |
| Edition: | 1st |
| Surface: | Clay |
| Location: | Veli Lošinj, Croatia |
| Venue: | Ljubicic Tennis Academy, Croatia |
| Champs: |  Carlos Taberner |
| Champd: |  Andrej Martin /  Tristan-Samuel Weissborn |
The 2021 Lošinj Open was a professional tennis tournament played on clay courts. It was the first edition of the tournament which was part of the 2021 ATP Challenger Tour. It took place in Lošinj, Croatia between 18 and 24 October 2021.
| Country | Player | Rank1 | Seed | |
|---|---|---|---|---|
 Italy | Marco Cecchinato | 82 | 1 | |
| Spain | Carlos Taberner | 116 | 2 | |
| Slovakia | Andrej Martin | 120 | 3 | |
 Serbia | Nikola Milojević | 136 | 4 | |
| Blaž Rola | 177 | 5 | ||
| Italy | Alessandro Giannessi | 185 | 6 | |
 United Kingdom | Jay Clarke | 195 | 7 | |
 France | Mathias Bourgue | 201 | 8 |
The following players received wildcards into the singles main draw:
Victor Vlad CorneaThe following players received entry into the singles main draw as alternates:
The following players received entry from the qualifying draw:
Nicolás Álvarez Varona
Arjun Kadhe
Georgii Kravchenko Filip MisolicSee main article: 2021 Lošinj Open – Singles.
Carlos Taberner def. Marco Cecchinato Walkover.See main article: 2021 Lošinj Open – Doubles.
Andrej Martin / Tristan-Samuel Weissborn def. Victor Vlad Cornea / 
Ergi Kırkın 6–1, 7–6(7–5).