Junya Koizumi (小泉 純也, Koizumi Jun'ya) (January 24, 1904 – August 10, 1969) was a Japanese politician who served as Director General of the Japan Defense Agency during the 1960s.

Koizumi was born Junya Samejima (鮫島 純也, Samejima Jun'ya) in Higashi-Kaseda, Kagoshima Prefecture (now part of Minami-Satsuma); his family were fishermen.

He attended high school at night while working in a department store, and then attended law classes at Nihon University while working as a secretary to a Diet member.

[1] Koizumi was purged from politics by the Allied occupation government in the late 1940s, but returned to the Diet in 1952.

He was a close ally of Nobusuke Kishi in the postwar years, served as Vice-Minister of Justice under Ichirō Hatoyama and became Director General of the Japan Defense Agency under Hayato Ikeda and Eisaku Satō.