Reporters Without Borders, accused Yahoo of helping Chinese authorities in jailing a pro-democracy activist, Jiang Liju.
According to what the international group dedicated to freedom of the press said on Wednesday this would be the third dissident sent to prison on information gathered from Yahoo by Chinese authorities.
Information gathered from Yahoo Web mail was used to convict Jiang Liju and sentence him to four years in prison for subversion in November 2003, the organization said.
Based on documents obtained by Reporters Without Borders, Yahoo Holdings in Hong Kong confirmed that the email account ZYMZd2002 had been used jointly by Jiang and Li Yibing, who is suspected of being a police informer.
The advocacy group called on Yahoo to remove its email servers from China.
Yahoo said it was unaware of the Jiang case, even though Reporters Without Borders claimed to have met with Yahoo executives on April 10.
"It is the only way to avoid taking part in the current crackdown against journalists and democrats," the Paris-based group said.
"Let us make clear that we condemn punishment of any activity internationally recognized as free expression, whether that punishment takes place in China or anywhere else in the world," Yahoo spokeswoman Mary Osako said in an email.
"While we absolutely believe companies have a responsibility to identify appropriate practices in each market in which they do business, we also think there is a vital role for government-to-government discussion of the larger issues involved," she added.