Patriarch Sergius (Russian: Патриарх Сергий; born Ivan Nikolayevich Stragorodsky, Иван Николаевич Страгородский; January 23 [O.S. January 11] 1867 – May 15, 1944) was the 12th Patriarch of Moscow and all the Rus', from September 8, 1943 until his death on May 15, 1944. He was also the de facto head of the Russian Orthodox Church in 1925–1943, firstly as deputy Patriarchal locum tenens (1925–1937) subsequently as Patriarchal locum tenens (1937–1943).

The expression Sergianism, which designates a policy of unconditional loyalty to the Soviet regime practised by the leadership of the Russian Orthodox Church, and is associated with his 1927 declaration, is derived from his name (see the Declaration of loyalty toward the USSR section below).