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.

The following players received wildcards into the singles main draw: The following players received entry into the singles main draw as alternates: The following players received entry from the qualifying draw: