| Date: | 2–8 August 2021 |
| Edition: | 2nd |
| Surface: | Hard |
| Location: | Concord, United States |
| Venue: | The Thoreau Club |
| Category: | WTA 125 |
| Prize Money: | $115,000 |
| Champs: |  Magdalena Fręch |
| Champd: |  Peangtarn Plipuech / Jessy Rompies |
| Before Year: | 2019 |
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.[1] The tournament could not be staged in 2020 due to COVID-19 pandemic.
| Country | Player | Rank1 | Seed | |
|---|---|---|---|---|
 Belgium | ||||
| Hsieh Su-wei | 73 | 2 | ||
| Madison Brengle | 75 | 3 | ||
 Russian Federation | Vera Zvonareva | 88 | 4 | |
| Lauren Davis | 94 | 5 | ||
| Christina McHale | 100 | 6 | ||
| Belgium | Greet Minnen | 110 | 7 | |
 Mexico | Renata Zarazúa | 137 | 8 | |
 Australia | Storm Sanders | 138 | 9 |
The following players received wildcards into the singles main draw:
Alycia Parks Katrina Scott Sophia WhittleThe following player received entry using a protected ranking:
The following players received entry from the qualifying draw:
The following player received entry as a lucky loser:
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| Country | Player | Country | Player | Rank1 | Seed | |
|---|---|---|---|---|---|---|
| Hsieh Su-wei | Hsieh Yu-chieh | 182 | 1 | |||
 India |  United Kingdom | |||||
 Ukraine | Kateryna Bondarenko | Tatjana Maria | 279 | 3 | ||
| Quinn Gleason | Jamie Loeb | 336 | 4 |
The following pair received a wildcard into the doubles main draw:
Brittany Collens / Victoria Hu Ankita Raina / Eden Silva → replaced by Mona Barthel / Ankita RainaSee main article: 2021 Thoreau Tennis Open – Singles.
Magdalena Fręch def. Renata Zarazúa 6–3, 7–6(7–4)See main article: 2021 Thoreau Tennis Open – Doubles.
Peangtarn Plipuech / Jessy Rompies def. Usue Maitane Arconada / Cristina Bucșa 3–6, 7–6(7–5), [10–8]