Siniaková was born to a Czech mother and Russian father Dmitry, a former boxer and her coach. She began playing on the ITF Women's Circuit in the Czech Republic in June 2012.
Kateřina Siniaková won her first WTA singles title at Shenzhen Open. The tournament began by defeating Peng Shuai in the first round in two sets. In the second, she edged world No. 4 Simona Halep in three sets (7–5 in the third) and claimed first win over a top-10 player. Her run continued by beating qualifier Nina Stojanovic and world No. 9 Johanna Konta in three sets. In the final, she defeated Alison Riske 6–3, 6–4 and won first WTA title.
After a little bit of struggling in tournaments not being able to win more than two matches in a row, she got back to Båstad for Swedish Open, where she reached her first career final in previous year. She eventually clinched her second career WTA title here, shocking top-seed and former world No. 1 Caroline Wozniacki, 6–3, 6–4 in the final.
In doubles, Siniakova together with Barbora Krejčíková won the women's doubles title at the French Open and Wimbledon.