Kim and Ahn are the co-founders of the New Politics Alliance for Democracy, which changed its name to the Minjoo Party of Korea on Wednesday.
Kim announced his departure on Sunday in a news conference at the National Assembly, saying that he is leaving the party to make a fresh start to secure victory in the general elections.
Kim vowed to dedicate himself in the creation a new political order that will win the elections.
Since Ahn left the party last month, the number of defections reached nine including Kim, reducing the party's seats to 118 from 127.
Kim will reportedly join Ahn's proposed new party to gather opposition forces.
We will analyse the fallout of his defection on the opposition party, with political commentator Jung Sik Seo.
