Promoted to lieutenant colonel in July 1945, Sejima was assigned to the staff of the Guandong (Kwantung) Army in Manchuria. Subsequent Soviet entry into the war on 10 August 1945 brought Sejima an 11-year imprisonment in a Soviet gulag in Siberia. During his internment, Sejima was summoned to the International Military Tribunal for the Far East in Tokyo as a witness for Japanese war crimes against the Soviet Union.
Released by the Soviets in 1956, Sejima built a successful career in business and became the president of the prestigious Ito Chu Company in 1978. From 1981 to 1986, he also served as unofficial political adviser to the cabinet of Nakasone Yasushiro. Sejima remained silent on his wartime record. His alleged negotiations with the Soviets in the immediate aftermath of the 1945 cease-fire concerning the use of Japanese prisoners of war as a labor force in Siberia as part of war reparations remain a matter of controversy. Tohmatsu Haruo
Hosaka, Masayasu. Sejima Ryuzo: Sanbo-no showashi [Sejima Ryuzo: A staff officer's Showa history]. Tokyo: Bungeishunju, 1987.; Nimmo, William F. Behind a Curtain of Silence: Japanese in Soviet Custody, 1945–1956. Wesport, CT: Greenwood Press, 1988.