Medical health officer Dr. Julie Kryzanowski says a student at Central Collegiate in Moose Jaw was diagnosed at the end of July.
She says about 160 students and staff at the school are being tested.
Kryzanowski says there are two other students who have active TB in Regina and testing is going on at three different schools.
She says each year there are about 80 cases of active TB in Saskatchewan.
The disease is treatable with antibiotics.
"Most of the cases of tuberculosis occur in the northern part of the province," Kryzanowski said. "We've had three in the southern part of the province (which) is a bit unusual, but it’s not outside of the normal numbers we see."
People who are infected and do not show symptoms are not contagious unless they have active TB.
Kryzanowski said only about 10 per cent of people who are infected go on to develop the active disease in their lifetime.
People with TB develop a cough, sometimes with blood, and can also have a fever, weight loss and night sweats