Lu Guang, who lives in New York, was invited to Xinjiang for a talk in October. His wife Xu Xiaoli says she last heard from him on 3 November.
Officials later told her that national security officers in the heavily-controlled region had taken Mr Lu away.
Ms Xu told the BBC that she did not know whether Mr Lu had done anything to provoke government anger.
Mr Lu is a three-time World Press Photo award winner who focuses on environmental and societal issues in China
On 23 October, Mr Lu flew to Urumqi – the capital city of the Xinjiang region – where he had been invited to attend a number of photography events.
He planned to fly to Sichuan where he would meet another friend, identified only as Mr Chen, on 5 November for a charity event.
But when Mr Chen arrived in Sichuan, he was unable to find Mr Lu and contacted Ms Xu to ask about his whereabouts.
She had no idea where he was as she hadn’t heard from him since 3 November. Ms Xu contacted the wife of the person who had invited her husband to Xinjiang and was told both Mr Lu and the host had been taken away by national security.
That was later confirmed by local officers from Zhejiang province, Mr Lu’s hometown.
They said they were unable to provide any more information.
“He has been lost for more than 20 days and as his most direct family member, I have not received any notice of his arrest,” Ms Xu said in a detailed letter posted on Twitter. “I have repeatedly contacted Xinjiang police but have been unable to get through.”
“It is our 20th wedding anniversary [next week]. We should be celebrating it together. I can only hope for his safe return.”
Xinjiang, in far western China, has become notorious for its tight security controls and heavy surveillance and police presence amid a widely criticised operation to tackle what it says is growing radicalism among the ethnic Uighur Muslim community.
The government is also sensitive to criticism and has detained reporters before who were investigating negative stories about China.