Swanson, 41, will play Marlowe Viccellio, a beautiful and mysterious woman who catches the attention of Lassiter (Timothy Omundson). Despite their love connection, Shawn (James Roday) and Gus (Dulé Hill) begin to suspect Marlowe of killing someone "vampire-style" after the police find a blood-drained body. The episode is appropriately titled "This Episode Sucks."
The appearance is a bit of role reversal for Swanson, who is best known for playing the original vampire slayer in the 1992 film version of Buffy, the Vampire Slayer. More recently, she made a cameo on One Tree Hill last year.
Psych will return for its sixth season this summer on USA.