Walter Joseph Ciszek, S.J. (November 4, 1904 – December 8, 1984) was a Polish-American Jesuit priest of the Russian Greek Catholic Church who conducted clandestine missionary work in the Soviet Union between 1939 and 1963.

Fifteen of these years were spent in confinement and hard labor in the Gulag, plus five preceding them in Moscow's infamous Lubyanka prison. He was released and returned to the United States in 1963, after which he wrote two books, including the memoir With God in Russia, and served as a spiritual director.

Since 1990, Ciszek's life has been under consideration by the Catholic Church for beatification. His current title is Servant of God.