Prime Minister Hwang Kyo-ahn is to become acting president, temporarily assuming presidential power while the constitutional court weighs the case for as long as 180 days.
The man who takes over as government caretaker in the wake of President Park Geun-hye's impeachment is seen by critics as a stiff and uncompromising defender of the fallen leader.
It would be an ignominious end for Park, who would go down in history as the first democratically elected South Korean president to be forced out of office.
Once called the "Queen of Elections" for her ability to pull off wins for her party, Park has been surrounded in the presidential Blue House in recent weeks by millions of South Koreans who have taken to the streets in protest.
Although the entire court bench was appointed by Park and her conservative predecessor, the strength of public anger towards her means she would be wrong to believe they could save her presidency. "Then, a constitutional court including justices appointed by Park would have to approve the impeachment".
Days before the start of Park's single five-year term, North Korea conducted its third nuclear test. Earlier this week, leaders of the ruling party said that they would encourage their members to vote their conscience in the secret ballot.
"A lot of attention and focus of the national media and public will be on the constitutional court", says James Kim of the Asan Institute for Policy Studies, a Seoul-based think tank.
He has indicated an interest in returning to South Korean politics after he leaves his United Nations post at the end of the year, but has recently refused to comment publicly on the Park scandal.
"Populism feeds into the Korean tradition of political street protests when politicians do stupid things", said Robert Kelly, a political science professor at South Korea's Pusan National University.
Before the vote, about 210 legislators were forecast to vote yes considering the number of ruling Saenuri Party lawmakers who are not loyal to President Park.
About 160 lawmakers affiliated with the two main opposition parties said Thursday they would resign en masse if parliament does not approve Park's impeachment, but that could be just a symbolic move because the parliamentary speaker won't likely approve them out of worries about further political chaos.
The scandal that has engulfed the president and paralysed her administration has focused on her friendship with long-time confidante Choi Soon-Sil.
The impeachment motion claims that Park gravely violated laws and the Constitution during her almost four years in office. While Park said she regretted hearing young people label their nation "hell" for economic hardships, Lee has roused crowds at rallies, calling for a "revolutionary change" to overhaul the economy.
The court could dismiss the impeachment move.
"President Park implemented or has begun to implement a number of policies that the USA really likes", said Bruce Klingner, a former Central Intelligence Agency deputy division chief for Korea, now at the Heritage Foundation, a conservative Washington think tank. Park says she will accept a direct investigation into her actions.
The president is also suspected of having put pressure on top Korean companies, including electronics giant Samsung, to donate to two foundations controlled by Choi.
Park has also been criticized for her handling of the sinking of the Sewol ferry, a disaster that killed more than 300 people, majority high school students.
Park, whose term is to end in early 2018, has said she would stand down if parliament arranges a stable power transfer.
While the historically large protests that helped push lawmakers to vote to impeach Park have been peaceful, the festive atmosphere kicked up a notch on Saturday as demonstrators let out relief that the president they so desperately want removed was finally halfway out the door. Park's lawyer calls the accusations groundless. Choi's father, a shadowy figure named Choi Tae-min who was a Buddhist monk, a religious cult leader and a Christian pastor at different times, emerged as Park's mentor.