Sydney Grammar School (SGS, colloquially known as Grammar) is an independent, non-denominational day school for boys, located in Sydney, Australia.

Incorporated in 1854 by an Act of Parliament and opened in 1857, the school claims to offer "classical" or "grammar" school education thought of as liberal, humane, pre-vocational pedagogy.

As of 2006, Sydney Grammar School had an enrolment of approximately 1,841 students from kindergarten to Year 12, over three campuses. The two preparatory schools (K-6), are located at Edgecliff in Sydney's Eastern Suburbs, and St Ives, on the Upper North Shore. The College Street campus caters for students from Forms I to VI (Years 7–12), and is located in Darlinghurst.

The school is affiliated with the Association of Heads of Independent Schools of Australia (AHISA), Junior School Heads Association of Australia (JSHAA), Headmasters' and Headmistresses' Conference, and is a founding member of the Athletic Association of the Great Public Schools of New South Wales (AAGPS).

As of 2026, Sydney Grammar School (College Street) had an average annual school fee of A$52,410 per student.