Tonkin will first appear in the third episode of the CW drama's fourth season, TV Guide reports.
The Australian actress will play an old friend of Tyler (Michael Trevino) - her character Hayley is described as "wild, sexy and very free-spirited".
23-year-old Tonkin played Faye Chamberlain on The Secret Circle until the show's cancellation in May.
She previously starred on Australian soap Home and Away and will next be seen in the sequel to 2010 film Tomorrow, When the War Began.
The Vampire Diaries returns to The CW on Thursday, October 11 at 8/7c. In the UK, the series airs on ITV2.