"Finally, I announce my resignation as JD-U Rajya Sabha member and from the primary membership of the party," he said at a function of his outfit Bihar Nav Nirman Morcha here.
Accusing Nitish Kumar of becoming a dictator and controlling both the party as well as the government, Kushwaha said there was no inner democracy.
He also hinted at forming a new political party soon.
Kushwaha, a leader from Koeri community, had closely worked with Nitish Kumar to oust Lalu Prasad's Rashtriya Janata Dal from power in Bihar in 2005. But later he developed differences with Nitish Kumar and declared open war against the chief minister.