Nitish Kumar sailed through with 126 votes in his favour after 91 Bharatiya Janata Party members and a Lok Janshakti Party (LJP) legislator walked out of the 243-member house.
Nitish Kumar got the support of his Janata Dal-United's (JD-U) 117 legislators, four independents, four of the Congress and the sole member of the Communist Party of India. One JD-U legislator is in jail.
Those who voted against Nitish Kumar included 22 members of the Rashtriya Janata Dal (RJD) and two independents.
Soon after the JD-U ended its 17-year-old alliance with the BJP Sunday, Nitish Kumar said he would seek a vote of confidence at a special session of the assembly today.
Nitish Kumar called off the alliance with the BJP and his colleague Sharad Yadav quit as convenor of the BJP-led National Democratic Alliance following differences with the BJP over the growing clout of Gujarat Chief Minister Narendra Modi.