| Champ: |  Aryna Sabalenka |
| Runner: |  Ashleigh Barty |
| Score: | 6–0, 3–6, 6–4 |
| Before Year: | 2019 |
| Draw: | 64 (12 Q/ 5 WC) |
| Seeds: | 16 |
See main article: 2021 Mutua Madrid Open.
Aryna Sabalenka defeated Ashleigh Barty in the final, 6–0, 3–6, 6–4 to win the women's singles tennis title at the 2021 Madrid Open. It was her 10th career WTA Tour singles title, and her first on clay.[1]
Kiki Bertens was the defending champion from when the tournament was last held in 2019,[2] but lost in the second round to Veronika Kudermetova.[3]
Paula Badosa became the first Spaniard in the tournament's history to reach the semifinals.[4]
The following are the seeded players. Seedings are based on WTA rankings as of 26 April 2021. Rankings and points before are as of 26 April 2021.
| Seed | Rank | Player | Points before | Points defending | Points won | Points after | Status | |
|---|---|---|---|---|---|---|---|---|
| 1 | 1 | Ashleigh Barty | 9,655 | 215 | 650 | 10,090 | Runner-up, lost to Aryna Sabalenka [5] | |
| 2 | 2 |  Naomi Osaka | 7,800 | 215 | 65 | 7,650 | ||
| 3 | 3 |  Simona Halep | 7,050 | 650 | 120 | 6,520 | Third round lost to  Elise Mertens [13] | |
| 4 | 5 |  Elina Svitolina | 5,835 | 10 | 10 | 5,835 | First round lost to Jil Teichmann | |
| 5 | 7 | Aryna Sabalenka | 5,205 | 10 | 1000 | 6,195 | Champion, defeated Ashleigh Barty [1] | |
| 6 | 9 |  Karolína Plíšková | 4,660 | 65 | 65 | 4,660 | Second round lost to  Anastasia Pavlyuchenkova | |
| 7 | 10 | Kiki Bertens | 4,405 | 1,000 | 65 | 3,470 | Second round lost to Veronika Kudermetova | |
| 8 | 11 | Belinda Bencic | 4,315 | 390 | 215 | 4,140 | Quarterfinals lost to  Paula Badosa [WC] | |
| 9 | 12 | Petra Kvitová | 4,160 | 215 | 215 | 4,160 | Quarterfinals lost to Ashleigh Barty [1] | |
| Garbiñe Muguruza | 4,120 | 10 | 0 | 4,110 | Withdrew due to a leg injury | |||
| 11 | 14 |  Jennifer Brady | 3,765 | (55)† | 120 | 3,830 | Third round lost to Anastasia Pavlyuchenkova | |
| 12 | 15 | Victoria Azarenka | 3,526 | 65 | 65 | 3,526 | Second round, withdrew due to a back injury | |
| 13 | 16 | Elise Mertens | 3,480 | 10 | 215 | 3,685 | Quarterfinals retired against Aryna Sabalenka [5] | |
| 14 | 17 |  Iga Świątek | 3,453 | (18)† | 120 | 3,555 | Third round lost to Ashleigh Barty [1] | |
| 15 | 18 |  Johanna Konta | 3,236 | 65 | 65 | 3,236 | Second round lost to Anastasija Sevastova [Q] | |
| 16 | 19 | Maria Sakkari | 3,020 | 10 | 120 | 3,130 | Third round lost to Karolína Muchová |
The following players received wildcards into the main draw:
The following players received entry using a protected ranking:
The following players received entry from the qualifying draw:
Irina-Camelia Begu Ana Bogdan Misaki Doi Kateryna Kozlova
Kristina Mladenovic Bernarda Pera
Nina Stojanović Ajla Tomljanović Vera ZvonarevaThe following players received entry as lucky losers:
Bianca Andreescu → replaced by 
Zheng Saisai Danielle Collins → replaced by Magda Linette Fiona Ferro → replaced by Elena Vesnina Sofia Kenin → replaced by Shelby Rogers Svetlana Kuznetsova → replaced by Polona Hercog Garbiñe Muguruza → replaced by 
Danka Kovinić Barbora Strýcová → replaced by 
Yaroslava Shvedova Sloane Stephens Serena Williams → replaced by Hsieh Su-wei Dayana Yastremska → replaced by Jeļena Ostapenko