Dong Guangping, 64, is being held incommunicado in Zhengzhou No 3 Detention Centre without charge or pending trial, according to his supporters.
Mr Dong, a former police officer-turned-activist, is known for standing against Chinese censorship and has been jailed three times previously by Beijing for speaking out against the government's human rights violations, including the Tiananmen Square crackdown.
The Toronto Association for Democracy in China in a statement said they have "credible information" regarding Mr Dong's detention, without mentioning the source of the confirmation.
"We've now had confirmation that Dong Guangping has indeed been handed over to Chinese authorities...Turns out he has likely been there since last October," Alex Neve, a senior fellow at the Graduate School of Public and International Affairs, University of Ottawa, told The Independent.
There has been no official acknowledgment of his detention, no family access or communication, and no contact with a lawyer, he added.
Mr Dong was arrested by the Vietnamese police on 24 August 2022 and since then there has been no information about his whereabouts.
Katherine Dong, the daughter of the activist, who lives in Toronto with her mother, said in November she feared her father had been handed over to Chinese authorities.
“It is beyond heartbreaking to learn that after my father’s courageous attempt to escape persecution in China – a perilous journey over the past three years that we had hoped would end with our happy reunion in Canada. He is, instead, once again behind bars in China," Katherine Dong said in a statement.
"I implore Chinese officials to relent in their cruel pursuit of my father simply because he stands strong for human rights.
"I beg them to free him and allow him to join us here in Canada."
Following his "enforced disappearance", the Vietnamese and Chinese officials have repeatedly refused to provide any information.
Katherine Dong, who is now a Canadian citizen, had appealed to the Justin Trudeau administration to help free her father, but the Vietnamese government refused to cooperate.
The activist escaped from China in January 2020, several months after he was released from serving his three-and-a-half-year jail term. He fled to Vietnam in 2020, where he had been in hiding, while his family moved to Toronto.
The Canadian government had reportedly issued Mr Dong travel documents and was negotiating with Hanoi to grant him permission to leave when he was arrested.
Three months after UN rapporteurs for human rights wrote to Vietnam, the government perfunctorily informed them that they had "no information regarding Dong Guangping".
"It is intolerable that Dong Guangping is in a Chinese prison for the fourth time in the past 20 years, simply because he has the courage to defend human rights," said Canadian senator Hassan Yussuff.
"While the relationship between Canada and China is strained, Canadian officials must pursue all avenues, including turning to other countries for assistance, to secure Dong Guangping’s freedom."