Renho, the former president of the Democratic Party, formally applied on Tuesday to join the Constitutional Democratic Party of Japan, with approval from the main opposition party expected soon.
On the same day, the CDP accepted applications from three other former Upper House members of the DP and from Shiori Yamao, the former DP policy chief who is now an independent member of the Lower House.
With Renho added to its ranks, the CDP will have 61 lawmakers across both houses of the Diet, making it the body's largest opposition force.
Renho, who goes by her given name, was one of the most popular DP members, known for her speech and debate prowess.
The DP, once the largest opposition party, was hit hard by an exodus of most of its Lower House members to Kibo no To (Party of Hope) shortly before the October general election.