What a New Prime Minister in Japan Could Mean for Relations with South Korea

Published September 25, 2025

The leadership contest to replace Shigeru Ishiba as Japan’s prime minister will not only decide the next head of the ruling Liberal Democratic Party (LDP) but could also reshape the trajectory of South Korea-Japan relations. On September 7, Prime Minister Ishiba announced that he would step down as head of the ruling LDP. “I have a truly painful sense of remorse at my own shortcomings,” he said at a press conference.

Although his government conducted successful trade negotiations with the Donald Trump administration, the LDP’s poor electoral showings in both the Tokyo municipal and House of Councillors elections made it difficult for him to remain in office. In South Korea, President Lee Jae Myung has sought to build on existing positive momentum with Japan, underscoring the importance of both U.S. and Japanese ties during his first diplomatic visit to Tokyo. But domestic pressures in both countries, coupled with the uncertainty of Japanese leadership, could complicate efforts to sustain this.

Although they had a short overlap in tenure, the current Korean and Japanese leaders have tried to build on the progress of their respective predecessors. Ahead of his visit to Washington, President Lee pointed out that he was the first democratically elected Korean president to visit Japan as their first diplomatic stop. “I think it shows the importance that we place on South Korea-U.S. relations and South Korea-Japan relations, and it has an important meaning,” he explained, adding similarly sanguine assessments in comments to the press.

According to a readout of the summit, Seoul and Tokyo agreed to continue trilateral security cooperation with the United States and identified potential policy areas of future cooperation, including “regional revitalization, declining birthrate and aging population, rapid population decline, agriculture, and ensuring resilience against disasters.” The readout also said they would expand the working holiday visa system to support people-to-people relations. The recent bilateral defense ministerial meeting in Seoul underlines how important the current leadership views maintaining the positive trend in bilateral relations.

So far in 2025, President Lee has upheld the pragmatic stance articulated on his presidential campaign, pledging not to overturn agreements made by his predecessors while urging long-term, cooperative dialogue with Japan that acknowledges historical grievances. Tokyo welcomed his remarks and expressed hope that his actions would follow suit.

The Contenders for Japan’s Leadership

Against this backdrop, the election of the next LDP leader could be a turning point in the bilateral relationship. The earliest candidate to make an announcement was Toshimitsu Motegi. He previously served as secretary general of the LDP, led the foreign and trade ministries under Shinzo Abe, and was the head negotiator with Robert Lighthizer, the U.S. trade representative during the first Trump administration. Another candidate who will stand for election is Minister of Agriculture, Forestry and Fisheries Shinjiro Koizumi. Although he was not successful in his last attempt at the top office, he is a rising star in the LDP and popular with the public. Finally, Japan may elect the first woman as prime minister in Sanae Takaichi. She held economic security and communications portfolios in previous cabinets, and was close to former Prime Minister Shinzo Abe.

Public polls paint an unclear picture of what will happen in October. Among these three candidates, polls conducted by both Kyodo News and Yomiuri Shimbun found that about a third of respondents said Takaichi was their pick for prime minister. But among LDP supporters, Koizumi is in the lead. LDP members will be the ones voting, which is good news for the Koizumi camp. But the shellacking of the ruling coalition in recent general elections means Takaichi’s popularity with the wider public may be more attractive in maintaining LDP electoral dominance.

Koizumi is the most likely to continue the upward trend in bilateral relations. In announcing his candidacy, he identified South Korea as “an important neighboring country” that makes it an important partner in addressing international issues. “The importance of South Korea-Japan relations and trilateral cooperation with the U.S. is growing,” he said, adding that he would continue his predecessors’ shuttle diplomacy with Seoul. Koizumi also has a familial reason to prioritize the Korean Peninsula. His father, Junichiro Koizumi, traveled to North Korea as prime minister and unveiled the Pyongyang Declaration in 2002, a significant milestone in the Japan-North Korea relationship.

Abe’s protege, Takaichi, has rhetorically moderated her stance on historical issues that often rankle Tokyo’s relations with Seoul. When asked about visiting the controversial Yasukuni Shrine, Takaichi said that she would “make an appropriate judgment,” a remark very different from what she said during the last LDP leadership election. These are all positive indicators that President Lee will find a constructive partner in whoever wins the LDP election.

Despite recent years of progress in bilateral relations, there are still challenges to maintaining this trend. On the Japanese side, for example, the Japanese government’s refusal to acknowledge that Koreans were forced to work in gold mines on Sado Island led South Korea to not participate in a memorial service for the second year in a row. On the South Korean side, domestic politics could make engagement with Japan difficult for President Lee. “Historically, South Korean left-leaning politicians have turned against Japan when they needed a boost from the public,” Japan expert Brad Glosserman recently observed. Progressives have already indicated their dissatisfaction with the Lee administration’s early decisions on Japan. “If Lee isn’t careful and lets Japan walk all over him, he may find himself turning into a Yoon-lite,” reads an editorial in the progressive Korean paper Hankyoreh.

The upcoming LDP leadership race will test whether Seoul and Tokyo can preserve the fragile progress made in recent years by balancing domestic politics with the importance of maintaining constructive bilateral relations.

Terrence Matsuo is a Non-Resident Fellow at the Korea Economic Institute of America. The views expressed here are the author’s alone.

Feature image from Shinjiro Koizumi’s official X account.

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