Liberal reformer Moon Jae-in was sworn in Wednesday after winning a snap election to replace impeached President Park Geun-hye.
Moon has advocated dialogue with North Korean leader Kim Jong Un, in stark contrast to Park's approach of tough sanctions and aggressive rhetoric.
Speaking at his swearing in ceremony, Moon promised to "resolve the security crisis as soon as possible."
"If it is necessary, I will fly immediately to Washington and also visit Beijing and Tokyo," he said.
"Under the right conditions, I will also go to Pyongyang. For peace on the Korean Peninsula, I will do everything that I can do."
Moon also vowed to further strengthen the alliance between South Korea and the US.
Return to sunshine?
A former special forces soldier and human rights lawyer, Moon came in for criticism during the campaign from hardline conservatives who saw him as weak on North Korea.
He has called for a combination of negotiations and economic cooperation alongside military and security measures.
"I am confident to lead the diplomatic efforts involving multiple parties, which will lead to the complete abandonment of the North Korean nuclear program, and bring the relationship between South and North to peace, economic cooperation and mutual prosperity," Moon said in an April 25 debate.
His stance has been compared to the so-called "Sunshine Policy" of the liberal governments of 1998 to 2008. By no coincidence, he was a key adviser to those administrations.
During the Sunshine Policy, Seoul actively engaged Pyongyang, which led to closer relations on both sides of the border and saw two South Korean Presidents visit the North Korean capital. However, the approach ultimately failed to halt North Korea's nuclear weapons program.
Weapons testing
Moon, who takes office Wednesday, is unlikely to get a long honeymoon when it comes to North Korea.
During the campaign, Moon advocated for engagement with North Korea -- particularly on the economic front -- as the best method to work towards a denuclearized Korean Peninsula.
Washington ties
The US and South Korea have a decades-long military and political alliance and Washington is by far Seoul's most important bilateral partner.
Facing criticism from the right that his party is anti-American, Moon has played up Trump's apparent willingness to meet with Kim, saying he is on the "same page" as the US leader.
However, one area where they firmly not in agreement is over the deployment in South Korea of the US Terminal High Altitude Area Defense (THAAD) missile defense system.
The caretaker administration which took over after Park's impeachment accelerated the THAAD roll-out, despite widespread criticism from Moon and others on the left, who have argued its deployment should be contingent on a vote in the country's National Assembly.
During their visits however, the US officials only met with caretaker President Hwang Kyo-ahn, who had already declared he would not stand to replace Park, and avoided any of her potential successors.
CNN's Paula Hancocks, Taehoon Lee, and K.J. Kwon contributed reporting.