The former co-stars will play husband and wife in the fifth episode of Season 7, according to TVLine, which first reported the news.
Supernatural executive producer Sera Gamble told TVLine the episode, titled "Shut Up, Dr. Phil," is best described as "'Bewitched gone wrong.'" Marsters plays a man whose life was perfect when he was first married to a witch (Carpenter). But now that he's angered her, he and the entire town are going to suffer the consequences.
Carpenter, who portrayed the self-centered Cordelia and was part of Buffy's Scooby gang, will next appear in the Sept. 1 episode of Burn Notice.
The new season of Supernatural premieres on Friday, Sept. 23 at 9/8c on The CW.