St. Theresa Catholic Secondary School (STCSS) is a Roman Catholic Secondary School in Belleville, Ontario, Canada. It operates under the direction of the Algonquin and Lakeshore Catholic District School Board. The school opened in 1998 and is named after the patron saint, St. Theresa (1873–1897).  The school has over 70 teaching staff, over 20 support staff and two administrators.  The approximately 750 students who attend St. Theresa are taught under the Ontario Secondary Curriculum.

St. Theresa is equipped with a range of athletic facilities. Because the school is located so close to the Quinte Health and Wellness Center, they are able to access the Wally Dever arena for their Hockey Program. In 2005, because of the growing student population a new wing was built, housing a new art studio, office and life skills room. The school property includes two soccer fields, a staff and student parking lot and a walkway to the Quinte Health and Wellness Center parking lot which provides students with access to further sports facilities.​

St. Theresa also has a number of off-campus classrooms that it shares with Sir James Whitney School, where welding, mechanics, cosmetology and construction programs are taught.  Within the school St. Theresa has many elective programs including Hospitality and Tourism, Dance, Drama, Art, Hockey and Physical Education.​​

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Faith                             

​St. Theresa Catholic Secondary School is a community of faith where learning, leadership, and sportsmanship are Christ-centered. The mission at St. Theresa is to inspire a love of learning and a respect for fellow human beings in an environment that fosters excellence in academics and spiritual development.