'Safer cells' at HMP Barlinnie in Glasgow contain features that could be used by prisoners attempting to take their own life, known as ligature points, a review of conditions at the jail found.
Chief Inspector of Prisons David Strang said a 2011 recommendation they be upgraded had not been met, an inspection in July found.
He welcomed a 17% fall in inmates at the over-capacity jail. He praised support for vulnerable prisoners at Barlinnie's Day Care Unit.
"Disappointingly, little progress has been made in ensuring that the safer cells are upgraded. This presents HMP Barlinnie and the SPS (Scottish Prison Service) with an unacceptable degree of risk," the inspector said.
He called the jail's reception area unfit for purpose.
HMP Barlinnie in Riddrie, in the north-east of Glasgow, was built in 1882 to house 1021 inmates but holds 1225.
A drop from 1477 in May 2011 is due in part to the opening of HMP Low Moss in Bishopbriggs.
Mr Strang said he fears the return of overcrowding with the closure of two prisons.
"While still operating beyond the design capacity, this drop in numbers has provided prisoners with greater access to a number of basic opportunities such as regular showers, education classes, library services, physical activities and more time out of cell," he said.
"I am, however, concerned this situation may deteriorate if, when HMP Aberdeen and HMP Peterhead close, HMP Barlinnie is required to provide the temporary accommodation for those prisoners displaced, pending the opening of the new prison at HMP Grampian."