The move has been widely seen as a significant attempt to take on , China's market leading search site, which had a second-quarter market share of 58.1%, compared to Google's 22.8% for the same period.
The size of Google's stake in is unknown, with local media reports in China ranging from 10% to 60%.
China is seen as a key area of expansion for Google -- it has the world's second largest internet market, after the US, with 162m web users.
Earlier this month, Google executives hinted that it planned to invest in one or two Chinese internet firms this year, with more to follow in 2008.
Tianya.cn, which includes blogs, classified advertising and photo hosting among its services, has more than 20m users and is among the top 100 sites in China.
This is Google's latest move into the growing social networking sector globally, following the purchase of video sharing site YouTube for £825m last year.
Previously, Google had owned a 2.6% stake in Baidu, but disposed of it in June last year as it wanted to focus on its own Chinese business ventures.
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