The network confirmed Wednesday that Meloni will be joining the popular series as an ancient, powerful vampire who controls the fate of the show's major characters.
The series' fifth season is expected to air next summer. Its stars include Anna Paquin and Stephen Moyer.
Meloni recently ended a 12-year run on NBC's "Law & Order: Special Victims Unit," where he played Detective Elliot Stabler.
Before that, the 50-year-old actor was a regular as a bisexual inmate on HBO's gritty prison drama, "Oz."