Daria Sergeyevna Kasatkina (born 7 May 1997) is a Russian-born Australian professional tennis player. She has been ranked as high as world No. 8 in singles by the Women's Tennis Association, achieved in October 2022. Kasatkina has won eight WTA Tour singles titles and one title in doubles. Her best results at the majors are reaching the semifinals at the 2022 French Open and the quarterfinals of the 2018 Wimbledon Championships.

Born in Russia to parents who were nationally ranked in athletics and ice hockey, Kasatkina began playing tennis at age six at the insistence of her older brother. She excelled as a junior, winning the European 16s championship and a junior major singles title at the 2014 French Open. Kasatkina quickly ascended the professional rankings, reaching No. 32 in the world while still 18 years old, and winning her first WTA Tour title in 2017 as a teenager at the Charleston Open. She rose to further prominence in 2018 by narrowly losing to fellow up-and-coming player Naomi Osaka at the Indian Wells Open in a match regarded as representing a new wave of women's tennis. Kasatkina also has won the biggest titles of her career at the Kremlin Cup and at the St. Petersburg Trophy at home in Russia. Following three successful seasons on the WTA Tour, Kasatkina struggled in 2019, falling into the bottom half of the top 100. However, she had a resurgent 2021, claiming two titles to return to the top 30, followed by another two titles in 2022, marking her return to the top 10.

In team competition, Kasatkina led the Russian team to victory at the 2020–21 Billie Jean King Cup, winning all her matches in the tournament. Alongside Anastasia Pavlyuchenkova, Liudmila Samsonova, Veronika Kudermetova, and Ekaterina Alexandrova, she helped secure Russia's first Billie Jean King Cup title since 2008. Following her coming out as lesbian in 2022 (as a Russian, where LGBTQ rights are severely restricted) and her public condemning of the Russian invasion of Ukraine, Kasatkina switched nationalities to Australia in 2025. At that time, she was the Russian No. 2 in women's singles; subsequently, she became the Australian No. 1.

Kasatkina is known for her crafty style of play and diverse shotmaking. Compared to hard-hitting players who rely on their physicality and pure power, she relies on her speed and 'tennis IQ' to outmanoeuvre her opponents.

In addition to playing tennis, Kasatkina hosts a popular vlog series on YouTube titled What The Vlog with her wife and Olympic figure skater Natalia Zabiiako. The channel covers life on the WTA circuit and often features cameos from other players.

Daria was born in Tolyatti, Samara Oblast to Tatyana Borisovna (née Timkovskaya) and Sergey Igorevich Kasatkin. Tolyatti is an industrial city located about 1000 km southeast of Moscow. Her father works as an engineer at the Volga Automotive Plant and her mother was a lawyer. Both of her parents were nationally ranked athletes in Russia (officially known as Candidates for Master of Sports); her mother in athletics, and her father in ice hockey. Kasatkina also has an older brother named Alexandr. Her brother had played tennis casually, and convinced her parents to have her also begin playing the sport when she was six years old. She initially played two to three times a week for two years. In time, she began competing in higher level tournaments. As the expenses to support her blossoming career began to rise, her parents made the difficult decision to sell their house to fund her training when she was about 12 years old. She is close childhood friends with fellow top players from the men's side Andrey Rublev and Alexander Bublik, as they are all the same age and grew up playing junior tournaments together in Russia.

As a junior, Kasatkina was ranked as high as No. 3 in the world. She began competing on the ITF Junior Circuit shortly after turning 14 years old and won her first title at just her second career event, the low-level Grade 4 Samara Cup. In early 2012 while still 14, Kasatkina won two higher-level Grade 2 tournaments in Moldova and France, the former of which was the first Grade 2 event she entered. Towards the end of the year, she helped Russia reach the final of the Junior Fed Cup alongside Elizaveta Kulichkova and Alina Silich, where they finished runners-up to the United States.

Kasatkina began excelling at the highest level junior tournaments in 2013. She reached her first Grade-1 final in doubles in January, which she followed up with her first Grade-1 final in singles in April. After failing to win a match at her only two Grade A events the previous year, Kasatkina finished runner-up to Belinda Bencic at the Trofeo Bonfiglio in May. She then won her first junior Grand Slam matches the following month, reaching the quarterfinals at the French Open. Following this event, she did not play another tournament until late August, when she won her first Grade-1 title at the International Hard Court Championship in the United States. Kasatkina's last event of the year was the Junior Fed Cup, where she played the No. 1 singles matches. With Veronika Kudermetova and Aleksandra Pospelova, the top-seeded Russian team won the tournament, defeating Australia in the final.