The ageing prison population means more prisoners need social care such as assistance with washing, dressing or feeding.
Ms Beal also said many had health issues and mobility problems which required wheelchairs or hoists.
Some have had operations such as hip replacements and need rehabilitation exercises, she said. Others have dementia or neurological issues.
“We were not built with an ageing population in mind,” Ms Beal said
“We don’t have enough space,” she said. “We don’t have enough accessible cells.
“We need to think about something like a secure care home.
“We have people whose needs are wider than a secure prison regime.”
Ms Beal said that a secure old people’s home would need a higher ratio of trained staff to each prisoner and would be expensive.
The amount the Scottish Prison Service spends on social care has already been increasing for the past five years.
Figures released to BBC Scotland under freedom of information show that £636,000 was spent in 2018 but this more than trebled to £2.1m in 2022.
Ms Beal said £500,000 of the prison social care budget was spent at her prison.