Seoul - Former human rights lawyer Moon Jae In was on Wednesday sworn in as South Korea's new president, hours after the country's election commission confirmed his victory in the polls.
Moon said he would "solve the current security challenges" and was willing to visit Pyongyang under "the right conditions."
"I will do my best to bring peace on the Korean peninsula," he added.
He said he would fly straight to Washington if required and would also go to Beijing and Tokyo.
Moon won 41 percent of the almost 33 million votes cast, the National Election Commission said.
His inauguration ceremony was held at the National Assembly in Seoul
"I will build a new nation. I will make a great Korea, a proud Korea. And I will be the proud president of such a proud nation," Moon was quoted as telling his supporters earlier.
Moon will be expected to deal with escalating tensions with neighbouring North Korea, after US President Donald Trump threatened to take unilateral action against the country for recent ballistic missile and nuclear tests.
The White House congratulated Moon on his win in a statement.
"We look forward to working with President-elect Moon to continue to strengthen the alliance between the United States and the Republic of Korea and to deepen the enduring friendship and partnership between our two countries," the statement said.
Tuesday's election was called after former president Park Geun Hye was impeached and forced to leave office over a corruption scandal.