A US state department spokesman said everyone held in China for "exercising their human rights" should be freed.
British artist Anish Kapoor said he was thrilled but urged supporters to "carry on making a noise".
Chinese state media said Mr Ai was bailed on Wednesday after pleading guilty to charges of tax evasion.
His arrest in April had prompted a global campaign for his release.
The 54-year-old told the BBC by phone earlier that he was back home and in good health.
"I am already home, released on bail, I can't talk to media but I am well, thanks for all the media attention," he said.
Mr Ai, who is an outspoken critic of the government, was detained as he boarded a Beijing flight bound for Hong Kong.
Most famous for helping design the Bird's Nest stadium that became the centre-piece for Beijing's 2008 Olympics, he had been held at a secret location without access to a lawyer.