The duo will appear in one episode of the show, TV Guide reports.
Phifer will star as Drake, a member of the a capella group which Gus (Dulé Hill) sang with when he was in college.
Marin will play Drake's mentor Deacon Jones. Drake arrives in town to honour Deacon, but Deacon is shot while accepting the award.
The a capella group, known as 'Blackapella', then try to help Gus and Shawn (James Roday) investigate the shooting.
Phifer is currently starring in Torchwood and has previously worked on shows including Lie To Me and ER. Meanwhile, Marin has appeared in Nash Bridges, Judging Amy, Lost and Off The Map.
John Rhys Davies, Cary Elwes, Greg Grunberg, Lindsay Sloane, Jennifer Finnigan and Jason Priestley are among the other stars who have recently signed up for appearances in Psych.
The show will return with its new season on October 12 on USA.
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