Mr Wu is connected with a book on Tiananmen
A spokesman for the US Embassy in Beijing said Wu Jianmin was released from jail in southern China and "has left the country... to be reunited with his family". He had no further details.
The spokesman said Wu "appeared in generally good health".
Correspondents say the release removes a key obstacle to gradually improving China-US relations ahead of a visit by President George W Bush to China in October.
Writer
Wu Jianmin, 46, was was arrested on 8 April in the southern city of Shenzhen.
He was accused of taking money from Taiwan spy organisations and entering the Chinese mainland to gather intelligence.
According to human rights activists, Mr Wu had frequently written about the suppression of the pro-democracy movement in China in 1989.
He is suspected of contributing to The Tiananmen Papers, a book on the discussions by China's top leadership before the 1989 massacre, in which hundreds, if not thousands, of pro-democracy protesters were shot dead
He taught at a Communist Party school and worked for a state newspaper in southern China from 1986 to 1988, said political activists. He is then thought to have left for the United States.
The embassy could not say Friday night if charges against Wu were dropped or if he had been tried and then expelled, as happened in July with three other scholars - one Chinese-born US citizen and two permanent US residents - accused of spying for Taiwan.
Taiwan split from China in 1949 but China regards it as a renegade province that must be re-united with the mainland.