"The US commitment to the Republic of Korea remains ironclad, and the US will continue to use the full range of diplomatic and military capabilities to uphold this commitment," he told South Korea's Defense Minister Song Young-moo, AP reported.
Mattis met with Song during a brief stopover on his way from China to Japan.
The US has 28,500 troops stationed in South Korea.
His stop carried a clear message of reassurance to South Korea following the sudden cancellation of a major joint military exercise planned for later this summer.
Mattis said the US commitment includes keeping "current US force levels" on the peninsula.
The military exercise was cancelled after US President Donald Trump's summit with North Korean leader Kim Jong Un.
Song said Kim's separate meetings with Trump and with South Korean President Moon Jae-in have laid the foundation for a permanent peace on the Korean Peninsula.
"If North Korea maintains its commitment to denuclearization and proves its willingness with actions, we will continue to devise measures to build mutual trust and establish peace," he said.