Patriarch Sergius 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 Patriarch of Moscow and all Rus', also known as the Patriarch of Moscow and all Russia, is the title of the primate of the Russian Orthodox Church. It is often preceded by the honorific "His Holiness". As the ordinary of the Diocese of Moscow, the office holder's direct canonical remit extends only to Moscow; however, as Patriarch, the office holder has a number of church-wide administrative powers as laid down by the charter of the Russian Orthodox Church.