An inmate and a prison worker were taken to hospital after three separate fires were started at a Bridgend prison.
Firefighters were called to HMP Parc at about 21:05 BST on Thursday after the juvenile prisoner set fire to a paper tissue in the young offenders' unit.
Two more fires - one in a cell and another involving clothing - happened just after 22:00 and 23:00.
A lighter was taken from the offender. Both were discharged from hospital.
Security firm G4S, which runs the prison, said: "The offender involved will face the consequences for breaking prison rules and our staff will work with him to try and prevent a repeat of this damaging behaviour."