Fr. Sergei Nikolaevich Bulgakov (Russian: Серге́й Никола́евич Булга́ков) (28 June [O.S. 16 June] 1871 - July 12, 1944) was a Russian Orthodox Christian theologian, philosopher and economist. Until 1922 he worked in Russia; afterwards in Paris.

Sergei Nicolaevich Bulgakov was born on 16 June 1871 Old Style to the family of an Orthodox priest (Nikolai Bulgakov) in the town of Livny, Oryol guberniya, in Russia.

He studied at Orel seminary, then at Yelets gimnasium. In 1894 he graduated from the Law School of Moscow University, where he had also undertaken a serious study of political economy.

During his study at the seminary Bulgakov became interested in Marxism and took part in the Legal Marxism movement. Studying Marxism, Bulgakov eventually became convinced of the impotence of this theory. Under the influence of works of Russian religious thinkers (Leo Tolstoy, Fyodor Dostoyevsky, Vladimir Solovyov, etc.), in the course of his meetings and arguments with Leo Tolstoy he found his religious beliefs again. He wrote a book about his evolution (Sergey Bulgakov, From Marxism to Idealism, 1903). Such an evolution was common for the Russian intelligentsia of the time, and he soon became one of their recognised ideologists. A primary contributor to the books Problems of Idealism (1902), Vekhi, Problems of Religion, About Vladimir Solovyev, About the Religion of Leo Tolstoy, The Religion of Solovyov's Philosophical Society, he participated in the journals New Way (Новый Путь) and Questions of Life (Вопросы Жизни). He was a leader of the publisher Way (Путь, 1911–1917), where he printed many important works of contemporary Orthodox theology.