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.
Early life Koizumi is a fourth-generation politician of the Koizumi family. His father, Junichiro Koizumi, was the Prime Minister of Japan from 2001 to 2006. His grandfather, Jun'ya Koizumi, was director general of the Japan Defense Agency (now Minister of Defense) and a member of the House of Representatives representing Kanagawa's 2nd District, a seat that Junichiro also held. His great-grandfather, Matajirō Koizumi, was Minister of Communications, mayor of Yokosuka and a member of the House of Peers.
Koizumi's parents divorced when he was a toddler. His father gained full custody of him and his elder brother Kōtarō. They were raised by one of Junichiro's sisters, Michiko. At that time joint custody was not permitted in Japan. After the divorce, Shinjirō was estranged from his mother, Kayoko, and did not meet her or his younger brother Yoshinaga again until he reached adulthood. Yoshinaga had not yet been born when their parents divorced, and was kept under the sole custody of Kayoko throughout his entire childhood.
Koizumi was born and raised in Yokosuka, his father's home district. When he was a student in junior high and senior high schools, he was engrossed in playing sports, especially baseball. He attended Kanto Gakuin University in Yokohama, receiving a Bachelor of Economics degree in 2004. He received his master's degree in political science from Columbia University. He spent one year as a part-time research fellow at the Center for Strategic and International Studies and as Young Leader of the Pacific Forum CSIS before returning to Japan in 2007.
Political career Member of the House of Representatives
Following his father's announced retirement in 2008, he was elected to his father's former seat representing the Kanagawa 11th district in the August 2009 general election, in which many other LDP seats were lost to the Democratic Party of Japan. He faced criticism for being a hereditary politician. He campaigned in a rented Toyota Prius with a volunteer staff.
Koizumi became head of the LDP's young legislators caucus in October 2011, a post previously held by Prime Ministers Takeshita, Uno, Kaifu, Abe and Asō. In February 2012, he started a project called "Team 11," which sent members of the division to areas of the Tōhoku region affected by the 2011 Tōhoku earthquake and tsunami on the 11th of each month to talk to locals and report back on the state of the reconstruction efforts. The group had 82 members, all under the age of 45, as of March 2013. Some observers compared the group to the powerful "Machimura faction" led by Nobutaka Machimura in terms of its political weight.
He was critical of the LDP under party president Sadakazu Tanigaki. In his first meeting as a party officer, he stated that "the image of the party is that it doesn't listen to the opinions of young people, has old thoughts and a hard head. That is why trust will not be restored." He argued in a November 2011 speech that the party's stance on the controversial Trans-Pacific Partnership trade agreement needed to be clarified. He later advocated breaking up the LDP's agreement with the Democratic Party of Japan and Komeito to pass an overhaul of the social security and tax system, directly arguing to Tanigaki that the party's mission should be to take down the DPJ government and to restore LDP control, and drawing comparisons to his father's maverick reputation.
Koizumi broke ranks with the LDP in August 2012 as one of seven LDP legislators who refused to walk out of the no-confidence vote instigated by Ichiro Ozawa against Prime Minister Yoshihiko Noda, in which the LDP and NK had agreed to throw out their votes. Although Koizumi voted for the no-confidence resolution, it was ultimately voted down 246–86. He voted for Shigeru Ishiba against Shinzō Abe in the LDP leadership election of September 2012, but did not make his vote public until after the election in order to avoid influencing others' votes.
Koizumi was re-elected in the December 2012 general election, which restored LDP control of the government under Abe. In the subsequent House of Councillors election in July 2013, he focused his campaigning efforts on disaster zones, outlying islands and areas in rapid population decline, giving speeches in support of their local LDP candidates. Kenichi Tokoi, a nonfiction author who wrote a book about Koizumi, said that his goal was to shake as many individual hands as possible and to leave the impression that he was kind enough to visit them, something which he could not achieve by campaigning in big cities. He led a delegation of the LDP's Youth Division during a trip to Taiwan in September 2013, meeting with President Ma Ying-jeou.
In October 2013, he was appointed Parliamentary Secretary in charge of Tohoku Recovery, in which capacity he would oversee post-disaster reconstruction efforts in Iwate Prefecture and Miyagi Prefecture. Ishiba, then secretary general of the LDP, stated that Koizumi "made a very strong case" with local disaster victims "about what he wanted to do and why." Tokoi characterized this posting as a test of Koizumi's administrative ability. Koizumi was re-elected for a third term in his Kanagawa 11th constituency in the 2014 general election.
Minister of Environment (2019–2021)
Koizumi was reportedly considered for a formal cabinet post under the Abe government in the reshuffles of October 2015 and July 2017. In October 2017, Koizumi was re-elected for a fourth time in the 2017 general election. On 11 September 2019, Prime Minister Abe appointed Koizumi as Minister of the Environment. He advocated for environmentalist policies, including ending Japan's use of nuclear and coal power, despite serving in a government considered skeptical of such policies.
However, he supports the construction of new coal-fired power stations in Japan, despite their particulate and greenhouse gas emissions. His support for the construction of two coal-fired power stations in Yokosuka has led him to be a "a target of activists' wrath".
In late August 2020, after the resignation of Shinzo Abe, Koizumi was named as a possible successor. A Kyodo News survey showed that almost 9% of those surveyed preferred him for prime minister, though some inside the party consider him too young to be in charge of the country. Koizumi declined to run and endorsed Minister of Defense Tarō Kōno for the position. After Kōno also declined to run Koizumi and Kono both endorsed Chief Cabinet Secretary Yoshihide Suga, a fellow Kanagawa politician. Suga ultimately won the LDP presidential election and became prime minister. Suga decided to retain Koizumi in his role. Poor approval ratings and criticism of the government's handling of the COVID-19 pandemic led to Suga announcing his resignation in September 2021. In the party's leadership election of 2021 (which ended with Fumio Kishida being elected as leader and later prime minister), Koizumi again endorsed Kōno for the position. When Kishida formed his first cabinet in October, Koizumi was replaced by Tsuyoshi Yamaguchi as Environment Minister.
From 2021 to 2024, Koizumi remained a backbencher in the Diet. Koizumi was returned to his seat in the House of Representatives for a fifth consecutive term during the 2021 general election, securing nearly 80% of the vote in his Kanagawa 11th district. He was chosen to chair the standing committee on national security in the House of Representatives in January 2024 after the previous chairman resigned in connection to the 2023–2024 Japanese slush fund scandal.
2024 LDP leadership election
On 14 August 2024, Prime Minister Fumio Kishida announced he would not seek a second term as LDP President, making the race an open field. By late August, both Kono and Ishiba had already declared their respective candidacies.
On 6 September, Koizumi officially announced his candidacy for the LDP presidency. In a press conference he pledged to introduce legislation that would legalize separate surnames for married couples and proposed holding a national referendum to determine whether or not Article 9 of the Constitution should be amended. Koizumi promised to dissolve the lower of house and call a general election “as soon as possible” if elected President and later Prime Minister. His candidacy was endorsed by former prime minister Yoshihide Suga. Koizumi gave his first street speech the following day in Tokyo's Ginza district. Koizumi, along with fellow front-runner Shigeru Ishiba, has been described as one of the "centrists" of the election.
Koizumi lost the election, placing third behind Sanae Takaichi and Shigeru Ishiba. Ishiba defeated Takaichi in a runoff election, having received support from Koizumi's camp. As LDP president, Ishiba appointed Koizumi chairman of the Election Strategy Committee, a senior party office. In the October 2024 general election Koizumi traveled across the country to rally support for LDP candidates, but the results were unfavourable, with the ruling coalition losing its majority. Koizumi, who had easily won re-election in his own seat, resigned as election strategy chairman to take responsibility. He was replaced by Seiji Kihara as the party's election chief. Koizumi was subsequently appointed as executive secretary to the LDP political reform headquarters.