Shinjirō Koizumi (Japanese: 小泉 進次郎, Hepburn: Koizumi Shinjirō; born 14 April 1981) is a Japanese politician serving as the Minister of Defense since October 2025. He previously served as the Minister of the Environment from September 2019 to October 2021 and Minister of Agriculture, Forestry and Fisheries from May to October 2025. He has also served as a Member of the House of Representatives for the Liberal Democratic Party (LDP), representing Kanagawa since 2009. He is the second son of former Prime Minister Junichiro Koizumi and the younger brother of actor Kotaro Koizumi. After graduating from Kanto Gakuin University and Columbia University, Koizumi worked as a researcher at the Center for Strategic and International Studies and became active politically as Young Leader of the Pacific Forum CSIS. He also spent time working as a political secretary for his father in the final years of his second term as prime minister. In the 2009 election, he was elected to the House of Representatives for the seat his father had occupied for more than 35 years. Koizumi was appointed as a Vice-Minister for Reconstruction after the election of Prime Minister Shinzo Abe in 2012, focusing on the northeastern region of Japan that was devastated by the March 2011 tsunami and subsequent nuclear disaster. He publicly opposed his father's calls for Japan to abandon nuclear energy immediately. In 2019, Abe appointed Koizumi to the cabinet as Minister of the Environment, a role he retained when Yoshihide Suga succeeded Abe as prime minister in September 2020. Koizumi endorsed Taro Kono in the 2021 LDP leadership election, which resulted in Fumio Kishida becoming prime minister. He subsequently left the cabinet and returned to backbench politics. Koizumi ran as a candidate in the 2024 LDP presidential election to succeed Kishida and placed third, losing the first round to Sanae Takaichi and Shigeru Ishiba, with the latter leading the final runoff. The LDP would lose its majority in the 2024 general election, after which Koizumi resigned as head of the party's Election Strategy committee. After working at the LDP's political reform headquarters, he was made Minister of Agriculture by Ishiba in May 2025. Following Ishiba's announcement of his resignation, Koizumi again ran as a candidate in the 2025 LDP presidential election, this time losing to Takaichi in the runoff. Takaichi subsequently appointed Koizumi as Minister of Defense.
Source: “Shinjirō Koizumi”, English Wikipedia (CC BY-SA 4.0), excerpt