Firefighters have been called to an incident at the Alexander Maconochie Centre.
A small fire started about 10.30am on Wednesday, an ACT Emergency Services Agency spokeswoman said.
The spokeswoman said the fire started inside the prison and crews had been able to see "smouldering".
Information on how the fire started is not known, but the spokeswoman said the fire was not believed to have been suspicious or started deliberately.
