Rishideo, a former minister, joined JD(U) in the presence of Chief Minister Nitish Kumar at his official 1 Anne Marg residence, they said.
He was a minister in the first NDA government in Bihar under Nitish Kumar. However, BJP had denied ticket to him in the last assembly election from Raniganj(SC) seat in Araria district.
State JD(U) president Bashist Narayan Singh, chairman of Mahadalit Commission Uday Kumar Manjhi and party MP RCP Singh were present at the programme.
JD(U) split from its ally BJP about three weeks ago after 17 long years over the question of elevation of Gujarat Chief Minister Narendra Modi to head that party's election campaign in the next Lok Sabha polls.