Sergius of Radonezh (Russian: Сергий Радонежский, romanized: Sergiy Radonezhsky; 14 May 1314 – 25 September 1392) was a Russian spiritual leader and monastic reformer.

He was the founder of the Trinity Lavra of St. Sergius near Moscow, what is now the most venerated monastic house in Russia.He exerted the greatest influence of any personage on the Russian Orthodox Church. Together with Seraphim of Sarov, he is one of the most highly venerated saints in Russia.

Historian Serge Aleksandrovich Zenkovsky wrote that Sergius, along with Epiphanius the Wise, Stephen of Perm, and the painter Andrei Rublev, signified "the Russian spiritual and cultural revival of the late fourteenth and early fifteenth century".