Jha joined the JD(U) in the presence of Chief Minister Nitish Kumar at his official residence. State JD(U) president Basistha Narayan Singh, minister Vijay Kumar Chaudhary and senior party leader Sanjay Jha were present on the occasion.
Jha, who had been associated with Jan Sangh and RSS since 1952 and twice served as Bihar BJP chief, was personally known to veteran BJP leaders Atal Bihari Vajpayee and L K Advani.
Later, Jha told reporters at JD(U) headquarters that he left BJP because he was not getting "respect" there.
He blamed former deputy chief minister Sushil Kumar Modi for his quitting the party and also for "widespread discontentment" in the party.
Lawyer-politician Jha had served as chairman of state Legislative Council during NDA rule from 2006 to 2012.
Asked if he would be able to do justice to his work in JD(U) having a mindset of a 'hinduatav' due to his long association with RSS, Jha said, "I am a lawyer and take brief from the client and work for him.