US-South Korea nuclear submarine deal: What does it mean?

The South Korean government announced that it has completed an agreement to jointly build nuclear-powered submarines with the United States.

The United States has approved “attack submarines” and agreed to cooperate on fuel supplies, according to a fact sheet released by the White House on Thursday.

The agreement marks a significant step in South Korea’s relations with the United States and comes at a time of rising tensions with nuclear-armed North Korea on the Korean Peninsula and expansionist China in the west.

Here’s what you need to know about the deal.

The agreement between the United States and South Korea comes after the leaders of both countries reached a broad trade agreement early last month that would reduce mutual tariffs from 25% to 15%.

US President Donald Trump imposed a 25% tariff rate on Seoul earlier this year; His counterpart Lee Jae Myung managed to reduce that rate to 15% after he said Seoul would invest $350bn (£265bn) in the US, including $200bn in cash investment and $150bn in shipbuilding.

In a White House statement released Thursday, the United States said it had “approved the Republic of Korea to build nuclear-powered attack submarines.” [and would] “We will work closely to develop the requirements for this project, including pathways to the fuel source.”

Trump said in an earlier post on social media platform Truth Social that the ships would be built at a shipyard in Philadelphia operated by South Korean conglomerate Hanwha.

Only six countries currently have nuclear-powered strategic submarines: the United States, China, Russia, the United Kingdom, France and India.

South Korea currently has about 20 submarines, but they are all diesel-powered and so must surface much more frequently. Nuclear submarines can also operate further and faster.

“I gave them the go-ahead to build a nuclear-powered submarine instead of the outdated and far less agile diesel-powered submarines they currently have,” Trump wrote on Truth Social.

South Korea is a powerhouse when it comes to civilian nuclear energy. It had a nuclear weapons program in the 1970s, but abandoned it under pressure from the United States.

And so the ability to enrich or reprocess uranium is limited by the United States because it is completely dependent on imports.

The latest ship program is aimed at countering North Korea, which recently announced it was pursuing its own nuclear submarine program.

President Lee told Trump at the APEC summit last month that South Korea needed them for this express purpose.

South Korean Defense Minister Ahn Gyu-back said in a television interview last week that nuclear submarines would be a “proud achievement” for South Korea and a major step in strengthening the country’s defenses against the North.

He added that the secrecy of nuclear submarines would keep North Korean leader Kim Jong Un “awake at night.”

North Korea also operates a nuclear submarine program, possibly with assistance from Russia, according to South Korean officials.

In March 2025, North Korea released photos of a nuclear-powered submarine it claimed was under construction, showing Kim visiting the shipyard.

Pyongyang is expected to have submarines within the next few years.

North Korea is also estimated to have an arsenal of approximately 50 nuclear weapons as part of its broader nuclear weapons program.

Jo Bee-yun, a research fellow at the Sejong Institute, suggested that Seoul’s purchase of nuclear submarines would help East Asia stay ahead of the escalating arms race.

“North Korea’s nuclear weapons are a proven fact,” he told the BBC. “[South Korea acquiring] “Nuclear submarines are just one step in a larger trend of increased tensions.”

It is unclear how much nuclear-powered submarines will add to South Korea’s defense capabilities; According to some experts, although they are very expensive, they do not change the balance of power on the Korean Peninsula.

Yang Uk, a research fellow at the Asan Institute for Policy Studies, told the BBC that the main purpose of the nuclear submarines was to reassure South Korean voters that their government was responding to North Korea’s nuclear threat.

“South Korea cannot develop its own nuclear weapons to counter North Korea,” Dr Yang said. “What can they do? Field nuclear submarines.”

Dr Yang believes North Korea may enjoy this change because it strengthens their case for having nuclear weapons; This means it will be harder to demand that Pyongyang give up its nuclear arsenal.

Jo nevertheless emphasized the strategic advantage South Korea could gain from the new submarine deal, calling it a “major change” that “means South Korea is now a regional player.”

“The best feature of a nuclear submarine is its speed,” he said. “Now it can go fast and forward, and South Korea can work together with more countries.”

According to Washington, support for South Korea’s nuclear-powered submarine program is likely aimed at putting pressure on both North Korea and China.

“Trump placed the burden of defense spending on South Korea,” Dr Yang said. “South Korea will greatly expand its defense budget. They will act as America’s proxy in putting pressure on China and North Korea.”

The United States and China have long competed for strategic influence in South Korea, forcing Seoul to walk a geopolitical tightrope. Recently, China has been increasing its naval activities near South Korea’s maritime border; This is a movement similar to those seen in the South China Sea.

Dr Yang said Beijing should be “furious” over South Korea’s nuclear submarine deal with the US.

Following the announcement of the agreement, Chinese Ambassador to South Korea Dai Bing said he hoped South Korea would “discreetly handle this issue, taking into account the concerns of all parties.”

Dai also added that Beijing had contacted Seoul on the issue through diplomatic channels, and emphasized that “the (security) situation on the Korean Peninsula and the region is still complex and sensitive.”

Although President Trump said that the submarines would be built in Philadelphia and would bring employment to the United States, South Korean officials argued that the submarines should be built locally and that existing facilities could deliver them in a much shorter time.

According to reports, South Korean Prime Minister Kim Min-seok himself told a parliamentary hearing that the South Korean-owned shipyard in Philadelphia “does not have the ability” to build such ships.

Hanwha, the owner of the shipyard, has not yet commented on the issue.

But now that an agreement has been reached, the next step will be to adjust the nuclear agreement between the two countries to allow the United States to provide nuclear fuel and set limits for its military use.