RNA - Family members and London-based group Al Qst, which is critical of the Saudi regime's human rights record, reported the death of Ahmed al-Amari, a former dean of the Islamic University in Medina, late on Sunday, Reuters reported.
Amari was arrested in August during a raid of his house, and was placed in solitary confinement.
His son, Abdullah, said funeral prayers would be held on Monday afternoon.
Prisoners of Conscience, another group promoting respect for human rights in the kingdom, said in a tweet last Sunday that he had suffered brain hemorrhage after Saudi officials injected him with poisonous substance during brutal torture in prison.
He and some others detained at the same time are believed to be close to influential religious scholar Safar al-Hawali, who was arrested in July 2018 after publishing a book critical of the Saudi royal family.
Saudi activists also reported on Friday that Nayef Ahmed al-Omran, an activist from the Shia-populated region of Qatif in Saudi Arabia's Eastern Province, had also died after eight years in detention.
Over the past years, Riyadh has redefined its anti-terrorism laws, using the legal basis to carry out sweeping raids against activists and outspoken clerics.
In January 2016, Saudi authorities executed Shia cleric, Sheikh Nimr Baqir al-Nimr, who would openly critique Riyadh's policies.
The kingdom, an absolute monarchy where public protests and political parties are banned, however, says it does not have political prisoners and denies torture allegations. Officials have said monitoring of activists is needed to ensure social stability.
The most high-profile incident involving Riyadh over the past year has been the murder of dissident journalist and Washington Post columnist Jamal Khashoggi last October. Khashoggi was killed and his body dismembered after he entered the Saudi Consulate in Istanbul. Various reports have verified that some on a team sent to carry out the murder had been drawn from among the personal bodyguards of Saudi Crown Prince Mohammad bin Salman, whom Khashoggi would repeatedly criticize before his murder.