Kim Han-gil, who co-led the New Politics Alliance for Democracy in 2014 with maverick politician Ahn Cheol-soo, also quit the party on Sunday.
His departure now pits the NPAD, which changed its name to the Minjoo Party of Korea last week, against a new political party being set up by Ahn, who already left the party last month.
At least 10 other NPAD lawmakers are expected to bolt with them.
Kim told reporters that the departure of both co-leaders of the NPAD "symbolizes" the doldrums the venerable party finds itself in. "New politics should be a clean slate. I will devote myself to forging a new political order," he said, adding that political infighting assured another election defeat for the NPAD.
Kim is widely expected to join Ahn's new party. The two met on Saturday, a day before Kim left the main opposition party. Kim is the ninth lawmaker to leave the NPAD since Ahn's exit and the 11th if two more lawmakers who bolted before Ahn are included.
That reduces the Minjoo Party's seats in the National Assembly from 127 to 118. With another 10 lawmakers expected to leave before Ahn launches his new party in February, the Minjoo Party may lose its status as the main opposition party.
Intense competition is expected in the general elections in April.
