Police and Emergency Services have been called to Yatala prison after inmates took control of a small area.
The incident happened just after 4pm on Tuesday but was resolved without injury by midnight.
It's unknown how many prisoners were involved in the peaceful 'sit-in' that saw a group refuse to return to their cells.
It's also believed 20 pizzas were delivered to the Northgate site but it's unclear if they were ordered by the men or used as part of negotiations.
Yatala is home to some of the State's most notorious prisoners including the Snowtown killers.
A criminal investigation is now underway.