Sevast'ian (or Sebastian) Moiseevich Tanatar (Ukrainian: Севастян Мойсейович Танатар; 7 (19) October 1849, Odesa – 30 November (19 December) 1917, Odesa) was a Ukrainian chemist of Karaite origin.
He later returned to Odesa University, where he achieved the rank of a Full Professor in 1896.
In 1895, he accomplished the transformation of cyclopropene into propylene at high temperature.
Tanatar also discovered compounds of hydrogen peroxide with Na2CO (1899), Na2SO4 (1901), and other salts and with urea (1908).
He also made significant contributions to nitrogen chemistry, such as industrial manufacturing of ammonia, nitrous acid and nitric acid, determination of the heat capacity and the atomic mass of beryllium, and the invention of gas mask.