Meloni is slated to portray "an ancient, powerful vampire who holds the fate of Bill [Stephen Moyer] and Eric [Alexander Skarsgard] in his hands," executive producer Alan Ball tells TVLine.com, who first reported the news.
The HBO series had been courting the Law & Order: SVU star for a major role since at least November. Meloni, 50, starred as Detective Elliot Stabler on SVU for 12 seasons before making his exit last spring. Since then, he's been noticeably absent from the screen, though he'll be starring as a general in the feature Man of Steel, due out in 2013.
The fifth season of True Blood is expected to return in summer 2012.