| Date: | 6–12 September |
| Edition: | 1st |
| Surface: | Clay |
| Location: | Tulln an der Donau, Austria |
| Champs: | Mats Moraing |
| Champd: | Dustin Brown /  Andrea Vavassori |
The 2021 NÖ 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 Tulln an der Donau, Austria between 6 and 12 September 2021.
| Country | Player | Rank1 | Seed | |
|---|---|---|---|---|
| Italy | Marco Cecchinato | 81 | 1 | |
 Czech Republic | Jiří Veselý | 90 | 2 | |
 Brazil | Thiago Monteiro | 93 | 3 | |
 Poland | Kamil Majchrzak | 114 | 4 | |
 Slovakia | Jozef Kovalík | 118 | 5 | |
 Bosnia and Herzegovina | Damir Džumhur | 120 | 6 | |
 Austria | Dennis Novak | 125 | 7 | |
 France | Hugo Gaston | 127 | 8 |
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:
See main article: 2021 NÖ Open – Singles.
Hugo Gaston 6–2, 6–1.See main article: 2021 NÖ Open – Doubles.
Andrea Vavassori def. Rafael Matos / Felipe Meligeni Alves 7–6(7–5), 6–1.