Thornhill Secondary School (TSS) is a public high school located in Markham, Ontario, Canada, in the Thornhill neighbourhood. Founded in 1954, the school is administered by the York Region District School Board.

Originally serving as an elementary school, TSS opened as a full-time high school in 1956 with 600 students. The TSS library was established in 1960 and renovated in 2007. In 1961, a technical and commercial wing was constructed. By 1962, the school had grown to 1,090 students, increasing its classrooms from 30 to 47 and adding 32 members to its original faculty of 29.

In 1976, the school underwent a major renovation; that same year, the drama program launched and the music program expanded. In 1999, an expanded science wing was constructed to accommodate an overflow of students. A new library and connecting hallway linking science to the technologies and music wings was completed in June 2007.

Alongside traditional disciplinary staples such as English, mathematics, science, physical education, and social science, Thornhill also offers courses in the fine arts, music, drama, business studies, and design and technology. The school offers Advanced Placement (AP) level subjects, aiming to challenge students with higher-order thinking skills.

The school also offers special education courses and classes — some integrated, some self-contained — for students with a variety of needs.

Thornhill Secondary School hosts a student newspaper, Eye of the Tiger, which reports on events in and around the school.

Thornhill Secondary School hosts a variety of athletic teams competing in York Region District School Board leagues.