Ichiro Ozawa, former deputy president of the Democratic Party of Japan, agreed Tuesday to assume the party's vice presidency.
The move came after party executives decided during a morning meeting to give the green light to DPJ President Katsuya Okada's proposal to tap Ozawa for the post, party members said.
Okada had informally asked Ozawa to rejoin the leadership to help bring unity to the party, DPJ sources said.
"To establish a united party, I will do as President Katsuya Okada says," Ozawa told reporters at the Diet.
Ozawa has held no post in the main opposition party since May, when Okada was first elected party leader after Ozawa withdrew his candidacy over his failure to join the national pension plan until it became mandatory in 1986.
Ozawa, who seemed to have kept a distance from Okada mainly over party management and security issues, remained without a post when Okada was re-elected party leader in September.
The DPJ currently has five vice presidents and Ozawa will be the sixth. He will mainly overlook issues related to elections and party management, the DPJ members said.
Ozawa was once an influential member of the Liberal Democratic Party. He is known for heavy-handed tactics and his preference to work behind the scenes. He is also viewed as having the kind of skills needed to lead the party to win elections.