Gegong Apang had resigned from the BJP last month.
Former Arunachal Pradesh chief minister Gegong Apang has joined the Janata Dal-Secular (JD-S), headed by former Prime Minister HD Deve Gowda.
Mr Apang said he joined the JDS because its a "secular" party. He had resigned from the Bharatiya Janata Party (BJP) last month.
In a letter to BJP president Amit Shah, he had expressed disappointment that BJP was not following former Prime Minister Atal Bihari Vajpayee'ss principle of "Raj Dharma".
He had alleged that the BJP has just become a platform to grab power.
Mr Apang had joined the BJP in 2014 after resigning from the Congress.