Helena Petrovna Blavatsky (Russian: Еле́на Петро́вна Блава́тская, Yelena Petrovna Blavatskaya, often known as Madame Blavatsky; née von Hahn; Ukrainian: Олена Петрівна Блаватська, Olena Petrivna Blavatska; 12 August [O.S. 31 July] 1831 – 8 May 1891) was a Russian philosopher and author who co-founded the Theosophical Society in 1875. She gained an international following as the leading theoretician of Theosophy.

Born into an aristocratic family of mixed Russian-German descent in Yekaterinoslav, then in the Russian Empire (now Dnipro in Ukraine), Blavatsky traveled widely around the empire as a child. Largely self-educated, she developed an interest in Western esotericism during her teenage years.