| 2021 Zagreb Open | |
|---|---|
| Date | 10–16 May 2021 |
| Edition | 17th |
| Surface | Clay |
| Location | Zagreb, Croatia |
| Champions | |
| Singles | |
 Sebastián Báez | |
| Doubles | |
 Evan King / Hunter Reese | |
The 2021 Zagreb Open was a professional tennis tournament played on clay courts. It was part of the 2021 ATP Challenger Tour. It took place in Zagreb, Croatia between 10 and 16 May 2021.
| Country | Player | Rank1 | Seed |
|---|---|---|---|
| ARG | Federico Coria | 92 | 1 |
 ESP | Pedro Martínez | 95 | 2 |
 SVK | Andrej Martin | 109 | 3 |
 JPN | Yasutaka Uchiyama | 110 | 4 |
 POR | Pedro Sousa | 113 | 5 |
 FRA | Benjamin Bonzi | 115 | 6 |
 BIH | Damir Džumhur | 118 | 7 |
 BRA | Thiago Seyboth Wild | 126 | 8 |
The following players received wildcards into the singles main draw:
Borna Gojo Nino Serdarušić Antun VidakThe following player received entry into the singles main draw as an alternate:
The following players received entry from the qualifying draw:
Sebastián Báez def. 
Juan Pablo Varillas 3–6, 6–3, 6–1. Evan King / Hunter Reese def. 
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