| Date: | 4–10 October |
| Edition: | 4th |
| Surface: | Clay |
| Location: | Barcelona, Spain |
| Champs: | Dimitar Kuzmanov |
| Champd: |  Harri Heliövaara /  Roman Jebavý |
| After Year: | 2024 |
The 2021 Sánchez-Casal Cup was a professional tennis tournament played on clay courts. It was the fourth edition of the tournament which was part of the 2021 ATP Challenger Tour. It took place in Barcelona, Spain between 4 and 10 October 2021.
| Country | Player | Rank1 | Seed | |
|---|---|---|---|---|
 France | Hugo Gaston | 116 | 1 | |
 Slovakia | Alex Molčan | 119 | 2 | |
 Spain | Bernabé Zapata Miralles | 120 | 3 | |
 Bosnia and Herzegovina | Damir Džumhur | 136 | 4 | |
 Serbia | Nikola Milojević | 141 | 5 | |
 Australia | Marc Polmans | 158 | 6 | |
| Czech Republic | Jiří Lehečka | 177 | 8 |
The following players received wildcards into the singles main draw:
The following player received entry into the singles main draw as an alternate:
The following players received entry from the qualifying draw:
Nicolás Álvarez Varona
Georgii Kravchenko Matteo Martineau
Yshai OlielThe following players received entry as lucky losers:
See main article: 2021 Sánchez-Casal Cup – Singles.
Hugo Gaston 6–3, 6–0.See main article: 2021 Sánchez-Casal Cup – Doubles.
Harri Heliövaara / Roman Jebavý def. Nuno Borges / Francisco Cabral 6–4, 6–3.