The information that was accessed included names, birth dates, identification numbers and addresses and Internet protocol addresses, Yonhap News Agency reported.
Users' passwords and registration numbers, the South Korean equivalent to a U.S. Social Security number, were not leaked, a presidential official, whose name was not reported, said.
Victims of the hacking can seek compensation, the official said.
Anonymous, a loosely associated network of hackers, has taken responsibility for the attack, which targeted a website for the presidential office, as well as another government agency and several media organizations on Tuesday.
Several North Korean websites were voluntarily taken offline on Tuesday after Anonymous warned of cyber attacks on the communist country.