Writer Yang Tongyan died yesterday, nearly three months after an August 23 surgery to remove a brain tumour, Amnesty International said in a statement, citing information from close friends.
Rights groups say a pattern has emerged in recent years where China releases activists from prison in poor health, or only weeks before they pass away, with late Nobel Peace Prize laureate Liu Xiaobo being a notable example this summer.
News of the death emerge as US President Donald Trump, whose government had urged China to release Liu before he died, arrived in Beijing for meetings with Chinese leader Xi Jinping.
"There seems to be no accountability for the pattern of deaths on medical parole of people labelled by the authorities as 'enemies of the state'," Bequelin added.
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Yang was convicted in 2006 for posting anti-government articles online, after having already spent a decade in prison for "counter-revolutionary" crimes.
The 56-year-old had been released from Nanjing Prison on medical parole in August following his diagnosis with an "aggressive form of brain cancer", PEN America said in a statement.
Yang was a 2008 recipient of PEN's Freedom to Write Award and a member of the Independent Chinese PEN Centre.
Liu was detained in 2008 after co-writing Charter 08, a petition calling for democratic reforms.
The veteran of the 1989 Tiananmen Square pro-democracy protests died in July, more than a month after he was transferred from prison to a heavily-guarded hospital to be treated for late-stage liver cancer.
Dissident writers and activists have long received lengthy prison sentences in China after speaking out about issues the authorities deem politically sensitive.
She died in custody after she was denied medical treatment, her family and lawyers said.
AFP tried to phone Yang's older sister, but an automated message said the number was not receiving incoming calls.
When asked whether Yang had requested overseas treatment prior to his death, foreign ministry spokeswoman Hua Chunying told AFP she did not have any information.
The Huashan hospital in Shanghai, where Yang was treated according to AFP sources, said their press department was not available to answer questions.