| Thornhill Secondary School | |
| Address: | 167 Dudley Avenue |
| Postcode: | L3T 2E5 |
| Country: | Canada |
| Coordinates: | 43.8097°N -79.4203°W |
| Motto: | Post Proelium Praemium - "After the Struggle, the Reward" |
| Religious Affiliation: | Secular |
| Founded: | 1954 |
| Superintendent: | Grant Fawthrop |
| Trustee: | Jenny Chen |
| Number: | 947792 |
| Principal: | Karen Fogleman |
| Grades: | 9-12, offers AP level subjects |
| Enrolment: | 823[1] |
| Enrolment As Of: | October 2021 |
| Colours: | Black and Yellow |
| Mascot: | Tiger |
| Newspaper: | Eye of the Tiger |
| Lastupdate: | February 2024 |
Thornhill Secondary School (TSS) is a secondary school in Thornhill, Ontario, Canada. Founded in 1955, the school is administered by the York Region District School Board system.
Originally it served as an elementary school, but in 1956 it opened as a full-time high school with 600 students. The TSS library was established in 1960 and renovated in 2007. In 1962, the school increased its classrooms from 30 to 47 and added 32 members to its original faculty of 29.
By 1962, 1090 students were enrolled. A technical and commercial wing was built in 1961. In 1976, the school undertook a major renovation. That same year, the drama program started and the music program expanded. In 1999, an expanded science wing was constructed to accommodate an overflow of students. A new library and school hallway connecting science to the technologies and music was completed in June 2007.