Correctional Service of Canada and Ontario Provincial Police are investigating the assaults of four inmates at Millhaven Institution.
The incident happened at the maximum-security federal institution on Tuesday.
The injured inmates were transported to an outside hospital for treatment and have all since returned to the institution.
No staff members or other inmates were injured.
In order to improve practices aimed at preventing this type of incident, Correctional Service of Canada says it will review the circumstances of the incident and take the appropriate measures.
