Four fire engines were called to HMP Birmingham, in Winson Green, where prison officers were on strike.
The prison officers were striking over pay
A handful of the 300 striking officers crossed the picket line to help tackle the fires which had broken out inside a cell and in rubbish outside.
One wing of the 1,450-inmate prison was also evacuated.
The fire service said the fires had already been extinguished by the time crews arrived. An inspection was carried out and there were no injuries.
A Prison Service spokeswoman said all prisoners had been returned to the wing and an investigation into how the fire started was underway.
The walkout by members of the Prison Officers' Association (POA) began on Wednesday morning, in defiance of a High Court injunction being granted to the Ministry of Justice.
It was called off in the evening after the POA was offered fresh talks by the government, the union said.