The cricketer-turned-politician sent his resignation letter to Fonseka, party sources said.
Ranatunga, 49, has not explained the reasons for his resignation and Fonseka was planning to write to him to ascertain reasons, the sources said.
Ranatunga, however, said he would continue to serve as an opposition member of parliament.
Ranatunga had left the ruling Sri Lanka Freedom Party of President Mahinda Rajapaksa in 2009 and joined hands with Fonseka, who had been jailed for running as an opposition candidate for the presidency in 2010.
Colombo: Sri Lanka's World Cup winning captain Arjuna Ranatunga on Friday quit his position as the deputy leader of the Democratic Party, led by former army chief Gen Sarath Fonseka.
First Published: Friday, November 09, 2012, 22:36