Thornhill Secondary School (TSS) is a secondary school in Thornhill, Ontario, Canada.

In 1962, the school increased its classrooms from 30 to 47 and added 32 members to its original faculty of 29.

In 1976, the school undertook a major renovation.

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.