2021 US Open – Girls' singles

Camila Osorio was the defending champion, having won the previous edition in 2019, but was no longer eligible to participate in junior events. She participated in the women's singles event, but lost to Ons Jabeur in the second round.

Robin Montgomery won the title, defeating Kristina Dmitruk in the final, 6–2, 6–4.

Seeds
All seeds received a bye into the second round. • 🇦🇩 Victoria Jiménez Kasintseva (quarterfinals)

• Alex Eala (quarterfinals)

• 🇷🇺 Diana Shnaider (second round)

• 🇪🇸 Ane Mintegi del Olmo (second round)

• 🇨🇿 Linda Fruhvirtová (third round)

• 🇧🇾 Kristina Dmitruk (final)

• 🇺🇸 Robin Montgomery (champion)

• 🇫🇷 Océane Babel (second round)

• 🇭🇺 Natália Szabanin (third round)

• 🇺🇸 Madison Sieg (second round)

• 🇩🇪 Mara Guth (second round)

• 🇺🇸 Elvina Kalieva (quarterfinals)

• 🇬🇧 Matilda Mutavdzic (third round)

• 🇵🇪 Dana Guzmán (second round, retired)

•  Michaela Laki (third round)

• 🇺🇸 Ashlyn Krueger (second round)

Seeds
• # 🇨🇿 Brenda Fruhvirtová (first round)

• # 🇷🇺 Mirra Andreeva (qualifying competition)

• # Lucija Ćirić Bagarić (qualified)

• # 🇺🇸 Valencia Xu (qualified)

• # 🇨🇦 Annabelle Xu (qualified)

• # 🇨🇦 Victoria Mboko (first round)

• # 🇹🇭 Pimrada Jattavapornvanit (qualified)

• # Chelsea Fontenel (first round)

• # 🇯🇵 Erika Matsuda (first round)

• # Yang Ya-yi (qualified)

• # 🇲🇽 Alejandra Cruz (qualifying competition)

• # 🇨🇦 Marina Stakusic (qualifying competition)

Qualifiers
• # Yang Ya-yi

• # 🇹🇭 Pimrada Jattavapornvanit

• # Lucija Ćirić Bagarić

• # 🇺🇸 Valencia Xu

• # 🇨🇦 Annabelle Xu

• # 🇺🇸 Theadora Rabman

Lucky loser
• # 🇺🇸 Katja Wiersholm