The 2021 Thoreau Tennis Open was a professional tennis tournament played on outdoor hard courts. It was the second edition of the tournament which is part of the 2021 WTA 125K series. The tournament had been upgraded from ITF 60K to WTA 125K, marking its first edition as a challenger event. It took place in Concord, United States between 2 and 8 August 2021. The tournament could not be staged in 2020 due to COVID-19 pandemic.
Singles main-draw entrants
Seeds
1 Rankings are as of 26 July 2021.
Other entrants
The following players received wildcards into the singles main draw: Alycia Parks Katrina Scott Lulu Sun Sophia Whittle
The following player received entry using a protected ranking: Rebecca Marino
The following players received entry from the qualifying draw: Robin Anderson Mariam Bolkvadze Victoria Duval Liang En-shuo
The following player received entry as a lucky loser: Alexa Glatch
Withdrawals
Before the tournament
Clara Burel → replaced by Rebecca Marino
Vitalia Diatchenko → replaced by Beatriz Haddad Maia
Anastasia Gasanova → replaced by Tatjana Maria
Nao Hibino → replaced by Kurumi Nara
Nuria Párrizas Díaz → replaced by Usue Maitane Arconada
Alison Van Uytvanck → replaced by Alexa Glatch
Doubles main-draw entrants
Seeds
1 Rankings are as of 26 July 2021.
Other entrants
The following pair received a wildcard into the doubles main draw: Brittany Collens / Victoria Hu
Withdrawals
Before the tournament
Ankita Raina / Eden Silva → replaced by Mona Barthel / Ankita Raina
Champions
Singles
Magdalena Fręch def. Renata Zarazúa 6–3, 7–6(7–4)
Doubles
Peangtarn Plipuech / Jessy Rompies def. Usue Maitane Arconada / Cristina Bucșa 3–6, 7–6(7–5), [10–8]