More than a dozen prison staff at HMP Parc in Bridgend are self-isolating after testing positive for Covid-19.
There are currently 13 members of staff self-isolating at home with coronavirus while one prisoner has tested positive and is isolating away from other prisoners.
A spokesman for HMP Parc said the prison was in "regular contact" with the staff in isolation and its director held regular briefings with staff to keep them up-to-date with government and health guidelines.
He said prisoners who tested positive for the virus were "immediately isolated" from others and their recent contacts were traced through an investigation.
The prison remained "very well staffed" and does not use agency workers, they added.
Parc Prison, which has 824 members of staff and 1,652 prisoners, is not currently classed as an outbreak site under Public Health Wales guidelines.
Parc Prison Deputy director Ian Coles said: “The health and safety of our staff and the people in our care is paramount. We have regular discussions with Public Health Wales on best practice, and we follow all national government guidance.
“We are proud of the way all our dedicated staff have responded to these challenging times.”
When shielding ended in Wales on August 16, staff considered vulnerable to Covid-19 were subject to a risk assessment on their return to work to introduce measures that would keep them safe. The risk assessments are repeated every four weeks.
Face coverings were introduced to the prison for prisoners and staff in October after Her Majesty's Prison and Probation Service guidance was updated.
