Nitish's appointment as JD-U chief was ratified at the party's national council meeting here in Nalanda district.
He was the sole candidate for the post after nominations were invited last month in Delhi and was declared "elected unopposed".
National council members from 23 states unanimously supported the resolution moved by Rajya Sabha member Haribansh to appoint the Bihar Chief Minister as JD-U chief.
Briefing reporters on the deliberations of the JD-U national council, the party's general secretary, KC Tyagi, said Nitish was authorised to explore the option of forging "a strong alternative" of non-BJP parties nationally to defeat the "communal forces", the way he did it in Bihar.
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Tyagi, accompanied by two senior Bihar ministers, Bijendra Prasad Yadav and Rajiv Ranjan Singh Lallan, said Nitish was the "most credible face" among a larger section of the people who saw him "with a hope to defeat the BJP and the RSS".
He, however, made it clear that the JD-U had "never declared" that Nitish would be the prime ministerial candidate in the 2019 Lok Sabha polls.