Hooded Man Kevin Hannaway has died after a short illness.
A native of west Belfast, the 77-year-old died in the early hours of Monday.
He was one of 14 Catholic men who say they were subjected to state-sanctioned torture when they were interned in 1971.
Some of the men were thrown from helicopters and told they were at high altitude despite being just feet from the ground.
The techniques used against the 14 men included being hooded, made to stand in stress positions, forced to listen to loud static noise and being deprived of sleep, food and water.
In 2021, the Supreme Court ruled that the methods used “would be characterised today” as torture.
None of the Hooded Men were convicted of any wrongdoing.
In June 2023 the PSNI apologised to the men over the treatment they received more than 50 years ago.
That came after one of the men, Joe Clarke, had earlier received a deathbed apology.
The group of men are also seeking an apology from the British government.
A lifelong republican, in 2018 Mr Hannaway was jailed for three years in 2018 by the Special Criminal Court in Dublin.
He was convicted of knowingly rendering assistance to the ‘IRA’, and assisting in interviewing people involved in ‘IRA’-organized activities in 2015.
Friend and Hooded Men justice campaigner Jim McIlmurray said the remaining Hooded Men were “saddened at the loss of yet another comrade”.
“At least he lived to see the Supreme Court in London concluding what he was subjected to was torture and not inhumane treatment,” he said.
“He didn’t live to see his apology from the British government, but hopefully his family will.”
In a statement “Saoradh, the IRPWA (Irish Republican Prisoners Welfare Association) and wider republican movement” offered “deepest sympathies” to the family of Mr Hannaway, who they described as a “former POW (Prisoner of War) and IRA volunteer”.
“Kevin was a tireless activist for the republican struggle, and the plight of republican prisoners and their families was to the fore for Kevin who himself had been imprisoned in recent years,” it said.
“A proud anti-imperialist, Kevin did not stand idly by in the face of occupation and oppression, he took the conscious decision to be a committed and dedicated revolutionary soldier.
“Right up until his death he was a volunteer of the Irish Republican Army.”
