An exit poll conducted jointly by three of South Korea's biggest broadcasters, which has generally been a reliable predictor of election results, showed Lee securing about 50% of the votes, giving him about a 10% lead over his conservative rival Kim Moon-soo.
Kim had recently served as the employment and labor minister in the Cabinet of former President Yoon Suk Yeol, who was impeached over his brief but shocking imposition of martial law late last year.
Lee's Democratic Party has been the primary opposition party for several years, but if he wins the South Korean presidency as predicted, the Democrats will find themselves back in control of both the executive branch and the parliament, where they already have a majority of seats, for the first time since 2022.
There has not been an elected president in office since Yoon was impeached late last year, so there will be no formal transition period. The inauguration ceremony for the 21st president of South Korea is set to be held Wednesday, just hours after the final votes were cast.
But Yoon's definition of an enemy within changed, as he first said the communists infiltrating South Korean politics were North Koreans, but then said it was Chinese influence that warranted his martial law declaration.
Many younger South Koreans bristled, and they came out in force in front of the parliament, urging lawmakers to impeach Yoon, who did maintain significant support, particularly among older and right-wing voters.
His huge political gamble with the martial law declaration, and his subsequent refusal to leave office following his impeachment, is widely seen as having shifted many voters to Lee and the Democratic Party.
Yoon beat Lee in the last presidential election in 2022, but by a razor-thin margin of only 247,077 votes.Lee is a veteran politician with 20 years of experience as an elected official.
He will rely on that experience to help him navigate the power struggle between South Korea's longtime closest ally, the United States, and its neighbor China, as Presidents Trump and Xi Jinping engage in a trade war that has affected global markets.
Lee vowed before the election that, if he won, he would work to further develop the U.S.-South Korea relationship "into a future-oriented, comprehensive strategic alliance."
Journalists gather inside the situation room for Democratic Party presidential candidate Lee Jae-myung, pictured on the poster, at the National Assembly, as voting in South Korea's presidential election comes to an end, in Seoul, June 3, 2025.
Lee also promised "stable management of Korea-China relations, promotion of Korea-Russia diplomacy," and to prioritize South Korea's national interests.
He has vowed to invest up to 100 trillion won (about $73 billion) to fund artificial intelligence work in the country, aiming to make South Korea one of the top three global AI powerhouses.
But Lee has offered little in the way of firm, specific commitments, and many South Koreans remained unclear about what kind of diplomatic policies he might adopt if and when he takes the country's highest office.
Before South Korea descended into internal political chaos in late 2024, the former conservative government had taken a hard line on North Korea and Kim Jong Un's isolated regime, fueling higher tension across the two countries' heavily militarized border.
If the election result does match the exit polls, Lee will take office facing multiple ongoing criminal cases, including a $1 billion property development scandal, alleged illegal remittances sent to North Korea, and the alleged misuse of government credit cards.
South Korean presidents do have a significant degree of immunity from criminal prosecution, though not from charges of treason or insurrection.
Opposition's Lee Jae-myung is "certain" to become president in South Korea's first election since impeachment chaos, according to the country's top broadcasters
Millions voted on Tuesday in a snap election triggered by former president Yoon Suk Yeol's impeachment over his failed martial law bid
Earlier polls also put Lee Jae-myung ahead of the ruling party's Kim Moon-soo, a former member of Yoon's cabinet
The election has been shaped by the fallout from Yoon's brief but disastrous move in December, which triggered divisive protests and months of political chaos
The new president faces a polarised country, businesses anxious over US tariffs and an unpredictable ally in Donald Trump
Counting has begun after polls closed at 20:00 local time (11:00 GMT). Results are expected to become clear around midnight
It is six months to the day that South Koreans fought off a military takeover by their former president Yoon Suk Yeol. Tonight, they have punished his party, putting the opposition back power.
The new president, Lee Jae-myung, fought this election as a referendum on martial law – promising to be the leader who would make sure this never happens again – and it worked.
Lee was once seen as a divisive character. He lost the last election three years ago and is facing multiple criminal charges. But over the past month he has managed to bring together people of all shades of politics, convinced that he is the only person who can steer South Korea back onto safe ground.
But before he can fix the country, Lee must deal with urgent challenges abroad. There are existential negotiations to be had with President Trump – not just about tariffs, but South Korea’s security. The US helps defend South Korea from the North, meaning there are thousands of troops stationed here, but Trump is reportedly considering removing some of them.
In the past Lee has been sceptical about Korea’s alliance with the US. His tone has softened slightly during this campaign, but at the same time he has said he wants to strengthen ties with China. This could cause friction between Seoul and Washington.
