Defense secretary James Mattis is visiting South Korea on the first foreign trip by a senior official in the Trump administration.
James Mattis is expected to use the visit to reassure Seoul of continuing US commitment to security deals in the face of threats from North Korea.
While campaigning, Donald Trump accused South Korea and Japan of not paying enough for US military support.
Donald Trump also suggested they could be allowed to arm themselves with nuclear weapons.
Both Japan and South Korea rejected this idea.
On the campaign trail, Donald Trump also said he was willing to talk to North Korean leader Kim Jong-un, contradicting longstanding US, South Korean and Japanese policy.
James Mattis will be in South Korea until February 3, and will hold talks with his Korean counterpart, Han Min-koo, among other officials.
The Pentagon said the visit would “underscore the commitment of the United States to our enduring alliances with Japan and the Republic of Korea, and further strengthen US-Japan-Republic of Korea security cooperation”.
James Mattis told reporters he would discuss the planned deployment of a US missile defence system in South Korea, and North Korea’s nuclear program.
His visit comes amid increasing threats from North Korea that it is ready to test-fire a new intercontinental ballistic missile at any time.
Under the Obama administration, the US and South Korea agreed to the deployment of a Terminal High Altitude Area Defense (THAAD) missile defense system to defend the South from North Korean missiles.
However, the move has angered China, which says it threatens its own security and goes “far beyond the defense needs of the Korean peninsula”.
There are just under 28,500 US military personnel based in South Korea, as part of a post-war arrangement. South Korea pays about $900 million annually towards the deployment.
On February 3, James Mattis will travel to Japan, for talks with Defense Minister Tomomi Inada.
There are a further 50,000 soldiers plus their dependents and support staff in Japan. The US pays about $5.5 billion for its Japanese bases in 2016, with Japan paying a further $4 billion.