"I am already home, released on bail, I can't talk to media but I am well, thanks for all the media attention," Ai, 54, said in an interview with the BBC.
Ai's detention had become a lightning rod for criticism of China's human rights record, The New York Times reported.
China's state-run Xinhua news agency reported Beijing police released Ai "on bail because of his good attitude in confessing his crimes as well as a chronic disease he suffers from."
Police said Ai had repeatedly said he was willing to pay the taxes he had evaded, Xinhua reported.
Ai was released under "qubao houshen," similar to bail, which generally means prosecutors will drop charges against a suspect on certain conditions, including good behavior, with monitoring during a period of time during which charges could be reintroduced, the Times said.
"This is a technique that the public security authorities sometimes use as a face-saving device to end controversial cases that are unwise or unnecessary for them to prosecute," Jerome A. Cohen, an academic expert on the Chinese legal system, said in an e-mail to the Times. "Often in such cases a compromise has been reached in negotiation with the suspect, as apparently it has been here."
Ai, a prominent critic of China's Communist Party leadership, was detained April 3 at the Beijing airport as he waited to board a flight for Hong Kong in what was widely viewed as an attempt to silence a prominent critic while authorities had time to decide on legal grounds for prosecuting him.