Tatyana Alexeyevna Afanasyeva (Russian: Татья́на Алексе́евна Афана́сьева) (Kiev, 19 November 1876 – Leiden, 14 April 1964) was a Russian / Dutch mathematician. On 21 December 1904 she was married to Paul Ehrenfest (1880 – 1933) an Austrian physicist and mathematician. They had two daughters and two sons: one daughter, Tatyana Pavlovna Ehrenfest, also became a mathematician.

Afanasyeva was born in Kiev in the Russian Empire (now in Ukraine). After her father died she was brought up by an uncle in St Petersburg, Russia, where she attended a women's pedagogical school and a Women's College. In 1902 she transferred to Göttingen, where she met Ehrenfest. The couple got married in 1904, and in 1907 they returned to St Petersburg. In 1912 they moved to Leiden, where Paul Ehrenfest was appointed to succeed H.A. Lorentz as professor at the University of Leiden.

Tatyana collaborated closely with her husband, most famously on their classic review of the statistical mechanics of Boltzmann. She published many papers on various topics such as randomness and entropy, and teaching geometry to children.