Reporters from the Associated Press saw more than 100 prisoners packed into one dark room, lined up shoulder to shoulder on the floor.
There is no electricity or ventilation, despite daytime temperatures well over 45C (110F).
The Iraqi officer in charge of the grim jail said it currently holds some 370 prisoners.
He says authorities were overwhelmed with detainees as Iraqi forces cleared the last neighborhoods of the city earlier this month at the end of a gruelling nine-month campaign.
He said: “Prisoners are infected with diseases, lots of health and skin problems, because they’re not exposed to the sun.
“The majority can’t walk. Their legs are swollen because they can’t move.”
More than 1,150 detainees have passed through the prison over the past three months, with 540 sent to Baghdad for further investigation, the officer said.
ISIS declared a Caliphate in Mosul, Iraq’s second city, after the death cult fanatics swept across northern and central Iraq in the summer of 2014.
As US-backed Iraqi forces battled block by block to retake the city, the jihadis rounded up civilians and used them as human shields.
Many fighters also fled the city by hiding among fleeing residents, complicating efforts by Iraqi forces to separate them from civilians.
Prisoners who were interviewed by the AP insisted they were innocent.
One, who said he was a civil servant, claimed: “You won’t find ten real (ISIS members) among these guys.
“And all of them have spent more than six months here. Since I got here eight months ago, I’ve only seen the sun once.”
He also claimed two prisoners had died in the packed holding cell, adding some prisoners “have pus coming out of their wounds. Once they go to the hospital, they come back with amputated legs or arms”.
Another inmate said: “We really want to die. None of us have received any visitors, relatives, family members. They don’t even know where we are.”
Harrowing images from liberated Mosul this week showed starving toddlers pulled from the ruins after their fanatic parents blew themselves up.
Yesterday a German schoolgirl who ran away to join ISIS was captured in Mosul, where she was reportedly part of an all-female sniper death squad.
And chilling documents found in the rubble reveal how ISIS bosses controlled the sale of women as sex slaves. – The Sun