64-year-old Kumar's name was proposed by RJD parliamentary board chief Rabri Devi and supported by Congress General Secretary C P Joshi at a joint legislature party meeting of the three parties.
Lalu Prasad, who is barred from fighting elections due to his conviction in a fodder scam case, was present at the meeting marked by bonhomie among MLAs of the three parties who shook hands and hugged each other.
Kumar would soon head for Raj Bhawan with senior leaders of constituent parties to formally stake claim to form government before Governor Ram Nath Kovind.
JD(U) spokesman Neeraj Kumar, who was present in the Grand Alliance legislature party meeting, told PTI that all the legislators approved the name of Kumar and clapped at the decision.
Also Read
RJD President Lalu Prasad, his JD(U) counterpart Sharad Yadav, Joshi and JD(U) General Secretary K C Tyagi were present, he said.
Earlier in the day JD(U) had formally elected Nitish Kumar as its legislature party leader.