The Canadian actor will play the recurring role of Professor Shane, according to TVLine.
The charismatic Shane is a believer in the occult and shares a connection with Bonnie (Kat Graham) and her family.
Vampire Diaries exec Julie Plec recently claimed that Bonnie is the "true hero" of the show.
"We're just seeing the beginning of growth for her, exploring who she is and what it means to be a witch and how she feels about it," said Plec.
New cast addition Alpay recently starred on Showtime's The Borgias and also filmed a role in rejected ABC pilot Americana.
Other stars signed up to appear on The Vampire Diaries include Phoebe Tonkin (The Secret Circle) and Grace Phipps (Fright Night).
The series returns to The CW in the US on Thursday, October 11 at 8/7c. In the UK, season four is expected to air on ITV2 in late 2012.
> Vampire Diaries star Paul Wesley: 'I didn't buy season three end'
> Vampire Diaries exec on Damon future: 'He'll kick ass, take names'
Watch Paul Wesley (Stefan) discuss The Vampire Diaries with Digital Spy below: