The party is headed by a former speaker of the lower house of parliament, the Mazhilis, Zharmakhan Tuyakbai, who confirmed the party's registration to RFE/RL's Kazakh Service.
"Yes, today Justice Ministry decided to registered [our] party and we received the registration papers," Tuyakbai said. "It took three long months [to get registered] -- longer than it should take."
Other opposition parties have also faced difficulties and delays in the registration process.
Tuyakbai ran unsuccessfully against incumbent President Nursultan Nazarbaev in the December 2005 presidential election as a candidate from the For A Just Kazakhstan political bloc.
The Social Democratic Party has some 140,000 registered members.