2021 US Open – Boys' singles

Jonáš Forejtek was the defending champion, having won the previous edition in 2019, but was no longer eligible to participate in junior events.

Daniel Rincón won the title, defeating Shang Juncheng in the final, 6–2, 7–6(8–6).

Seeds
All seeds received a bye into the second round. • 🇨🇳 Shang Juncheng (final)

• 🇺🇸 Samir Banerjee (quarterfinals)

• 🇪🇸 Daniel Rincón (champion)

• 🇺🇸 Bruno Kuzuhara (second round)

• 🇬🇧 Jack Pinnington Jones (second round, retired)

• 🇺🇸 Victor Lilov (quarterfinals)

• 🇺🇦 Viacheslav Bielinskyi (third round)

• Jérôme Kym (semifinals)

• 🇪🇪 Mark Lajal (third round)

• 🇫🇷 Sean Cuenin (second round)

• 🇫🇷 Sascha Gueymard Wayenburg (semifinals)

• 🇵🇪 Gonzalo Bueno (second round)

• 🇺🇸 Dali Blanch (second round)

• 🇪🇸 Alejandro Manzanera Pertusa (second round)

• 🇵🇱 Maks Kaśnikowski (third round)

• 🇺🇸 Alexander Bernard (third round)

Seeds
• # 🇲🇰 Kalin Ivanovski (first round)

• # 🇭🇰 Coleman Wong (qualifying competition)

• # 🇱🇹 Vilius Gaubas (qualifying competition)

• # 🇨🇿 Adam Jurajda (first round)

• # 🇨🇿 Matthew William Donald (qualifying competition)

• # 🇨🇿 Jakub Menšík (qualified)

• # 🇩🇪 Philip Florig (qualified)

• # 🇺🇦 German Samofalov (first round)

• # 🇧🇷 João Victor Couto Loureiro (qualifying competition)

• # Miguel Gomes (qualified)

• # Sebastian Dominko (qualified)

• # Elmer Møller (first round)

Qualifiers
• # 🇩🇪 Philip Florig

• # 🇺🇸 Kyle Kang

• # Miguel Gomes

• # 🇵🇪 Christopher Li

• # Sebastian Dominko

• # 🇨🇿 Jakub Menšík