China Frees Rebiya Kadeer
Mrs Kadeer, a successful businesswoman, was given eight years' in jail in 2000 for sending press cuttings overseas, "endangering national security".
The release comes ahead of US Secretary of State Condoleezza Rice's arrival in Beijing on Sunday.
The US has said that resolving the case of the Chinese Muslim businesswoman was one of its highest priorities in discussions with China over its human rights record.
Although the release was officially granted for medical reasons, it is thought to have been prompted by Ms Rice's arrival, the BBC's Tony Cheng in Beijing says.
The release was announced as the US said it would not seek to censure China at the annual meeting of the UN commission on human rights in Geneva.
"There have been improvements. We have seen progress in some of the things that we have been working on [with China]," a US spokeswoman said, without giving any further details.
Mrs Kadeer boarded a United Airlines flight in Beijing, the Dui Hua Foundation's statement said.
It said she was accompanied by an official of the US government.
The Dui Hua Foundation's president, John Kamm, directly linked the release with Mr Rice's visit to China.
"I've been led to believe... that the Chinese are doing this as a gesture for Condi Rice," Mr Kamm was quoted as saying by the Associated Press news agency.
"I was told they wanted to do this before she arrived," he said.
A prominent member of the Uighur ethnic minority in China's north-west Xinjiang province, Mrs Kadeer was charged with passing information to foreigners, separatism and attempting to overthrow the state.
According to transcripts, her trial centred on local newspaper clippings on the treatment of Uighurs that she had sent to her husband in the US.
China is intolerant of any dissent in the sensitive border region of Xinjiang, and is worried about several groups it brands as separatists.
Before her arrest, Mrs Kadeer had been held up as a role model to China's large Muslim population.
She had owned a prosperous department store and started a charity helping other Muslim women find work.
She had even been appointed to a seat on one of the Chinese government's highest consultative bodies.
The rights group Human Rights Watch said it was "thrilled" about Mrs Kadeer's release, but stressed that China operated a "revolving door" for political prisoners.
"The fact that the Chinese government releases people that never should have been in prison in the first place does not mean human rights are improving in the country," said Mickey Spiegel, senior researcher of the group's Asia division.
Source: BBC News