"I'm delighted to return for the series finale," Rosenbaum said in a statement. "I'm simply doing it for all of the fans out there who made Smallville the great success it is. I appreciate all of their passion, their relentlessness, and even their threats. Ha ha."
A huge topic of speculation and fan fodder, Rosenbaum's return comes as a sigh of relief for the Smallville diehards, who hoped he would honor the fans' wishes and come back to the series one last time.
As for how Lex Luthor will return in the two-hour season finale, airing May 13, executive producers Brian Peterson and Kelly Souders say, "There'll be no doubt about how Lex becomes the great rival in Clark Kent's life. He is the villain of the story."
Though a Lex Luthor has appeared on the series in the tenth and final season as a clone, Rosenbaum hasn't been seen on Smallville since the Season 7 finale. He'll next appear in the Fox comedy Breaking In.
TVLine.com first reported the news.