With parliament's impeachment vote against Yoon passed after his short-lived attempt to impose martial law, Yoon is suspended from exercising presidential powers, and the constitution requires the prime minister to take over in an acting role.

In a country sharply divided by partisan rhetoric, Han has been a rare official whose varied career transcended party lines.

He faces a challenging task of keeping government functioning through its gravest political crisis in four decades, while also dealing with threats from nuclear-armed neighbour North Korea, and a slowing economy at home.

His tenure as acting president could also be threatened by criminal investigations into his role in the martial law decision.

Han, 75, has served in leadership positions for more than three decades under five different presidents, both conservative and liberal.

His roles have included ambassador to the United States, finance minister, trade minister, presidential secretary for policy coordination, prime minister, ambassador to the OECD, and head of various think-tanks and organisations.

With a Harvard doctorate in economics, Han's expertise in the economy, trade and diplomacy as well as a reputation for rationality, moderate demeanour and hard work has made him a regular go-to man in South Korean politics.

Han has been prime minister since Yoon's term began in 2022, his second time serving in the role after a stint as prime minister under former president Roh Moo-hyun in 2007-2008.

"He has served in key posts in state affairs solely through recognition of his skills and expertise, unrelated to political factions," Yoon said when appointing Han in 2022, echoing words used to describe him when previous administrations tapped him for key positions.

"I think Han is the right candidate to carry out national affairs while overseeing and coordinating the Cabinet, with a wealth of experience that encompasses public and private sectors."

Han has experience working with South Korea's key ally the United States, having been deeply involved in the process of signing the U.S.-South Korea Free Trade Agreement.

Fluent in English, he was appointed South Korea's ambassador to the United States in 2009, working in Washington at a time when current U.S. President Joe Biden was vice president, and contributed to Congress approving the Free Trade Agreement in 2011.

Han has also served as board member of S-Oil (010950.KS), a South Korean refining unit of Saudi Aramco.

"He is a civil servant through and through who didn't take on a political colour despite working under (five presidents)," said a former high-ranking government official who declined to be identified.

Han's role in leadership is expected to last for months until the Constitutional Court decides whether to remove Yoon or restore his powers. If Yoon is removed, a presidential election must be held in 60 days, until which Han will stay at the helm.

The main opposition Democratic Party has filed a complaint against Han to be included in the investigations for failing to block Yoon's attempt at martial law.

If parliament decides to impeach Han, the finance minister is next in line among cabinet members to serve as acting president.

South Korea's Constitution does not specify how much the prime minister is empowered to do in carrying out the leadership role.

Most scholars say the prime minister must exercise limited authority to the extent of preventing paralysis of state affairs and no more, although some say he can exercise all the powers of the president, as the constitution put no restrictions.