Foreign Ministry spokeswoman Han Hye-jin said the activists returned home Friday after a diplomatic push by Seoul.
Yonhap News Agency says China arrested the activists while they were discussing ways to help North Korean defectors in China. Yonhap cited no sources.
Last week, China's public security minister met with South Korea's foreign minister in Seoul. Han says the meeting facilitated the release.
The 2010 U.S. State Department Human Rights Report says that non-governmental organizations estimate there could be up to hundreds of thousands of North Korean defectors in China.
Beijing has a treaty with Pyongyang that requires China to repatriate North Koreans who enter the country illegally.