The Save The Last Dance actress - who was linked to the show last month - will appear as a "mysterious young woman who forms a unique relationship" with Dexter (Michael C. Hall) following his wife's death.
She is scheduled to appear in ten episodes in the fifth season, which begins production later this month.
Julie Benz - who plays Dexter's murdered wife Rita - recently confirmed that her character's "presence is definitely going to be felt throughout the season".
"You can't deny that she had an effect on Dexter's life, which is really what this season's about," she told TV Guide, adding that the new season will explore more about her relationship with Dexter.
The fifth season of Dexter premieres this fall on Showtime.