SERGII OF RADONEZH (1322?–1392) was a Russian Orthodox monastic saint and founder of Holy Trinity-Saint Sergii Monastery (in present-day Sergiyev Posad). The life of Sergii is known largely from two fifteenth-century hagiographical accounts, supplemented by Russian medieval chronicles. Sergii himself wrote nothing.

Sergii (in secular life known as Bartholomew) was born in the principality of Rostov, but early moved with his family to the Muscovite village of Radonezh. In search of the ascetic life, he persuaded his already tonsured brother Stephen to venture with him into the neighboring forests. The severity of their life as hermits caused Stephen to withdraw within the year. For the succeeding two or three years (c. 1345–1348), Sergii tested his vocation alone. However, news of the solitary spread, and he attracted a company of independent monks around the wooden Trinity Church he had erected with his brother. In about 1353 Sergii accepted...