Koizumi Matajirō (コイズミ マタジロン; Mutsuura, Musashi Province, June 10, 1865 – Tokyo, September 24, 1951) was a Japanese politician and cabinet minister. He was the leader of a criminal organization in Kanagawa prior joining the Japanese Nationalist Party in 1919 and has a large tattoo of a red dragon which covered most of his back and upper arms. During the Second Japanese Civil War, Koizumi used his criminal network as one of major financial supporters of the Nationalist Party.
Along with Inukai Tsuyoshi, Koizumi was considered as one of the respected Nationalist Party elders, representing the party's right-wing faction against the left-leaning party elders, including Ozaki Yukio and Abe Isoo. He became the members of Nationalist Party's General Political Office from 1919 until his death in 1951.
Koizumi served as the first Speaker of the Legislative Council of the Republic of Japan from September 1, 1921 to September 1, 1931. Following the anti-organized crime campaign in 1930s under the administrations of Nakano Seigo and Suzuki Bunji, Koizumi was purged from the government in 1932 and from the party in 1934, thus led to his partial retirement from political life. However, in 1941, Koizumi returned to the public life after being appointed by President Nagayama Yoshida into the Supreme War Council until 1945. He then retired completely from politics in 1946 after failed to get official Nationalist nomination as one of the candidates for the National Congress elections representing Tokyo.