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

• Rankings are as of 3 May 2021.^[1]

The following players received wildcards into the singles main draw:

• Borna Gojo
• Nino Serdarušić
• Antun Vidak

The following player received entry into the singles main draw as an alternate:

• Dmitry Popko

The following players received entry from the qualifying draw:

• Sebastián Báez
• Marcelo Tomás Barrios Vera
• Ernests Gulbis
• Ramkumar Ramanathan

• Sebastián Báez def. Juan Pablo Varillas 3–6, 6–3, 6–1.

• Evan King / Hunter Reese def. Andrey Golubev / Aleksandr Nedovyesov 6–2, 7–6^(7–4).