2021 Lisboa Belém Open – Men's singles

Jaume Munar was the defending champion but chose not to defend his title.

Dmitry Popko won the title after defeating Andrea Pellegrino 6–2, 6–4 in the final.

Seeds
• # 🇧🇷 Thiago Monteiro (second round)

• # 🇯🇵 Taro Daniel (first round)

• # 🇫🇷 Hugo Gaston (semifinals)

• # 🇸🇰 Andrej Martin (semifinals, retired)

• # 🇷🇸 Nikola Milojević (quarterfinals)

• # 🇩🇪 Cedrik-Marcel Stebe (quarterfinals, retired)

• # 🇮🇹 Alessandro Giannessi (withdrew)

• # 🇰🇿 Dmitry Popko (champion)