The Iron Lady star will appear as centaur Chiron in the sequel to Percy Jackson & the Olympians: The Lightning Thief, according to The Hollywood Reporter.
The character was portrayed by Pierce Brosnan in the previous film.
The second movie sees Jackson (Logan Lerman) set out on a quest with his fellow demi-god friends to retrieve the legendary Golden Fleece.
Head is best known for his role as Giles in Buffy the Vampire Slayer, also portraying Uther Pendragon in the Merlin television series.
He joins Mary Birdsong, Yvette Nicole Brown, Missi Pyle and fellow Joss Whedon alumni Nathan Fillion in the Thor Freudenthal film.
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The franchise is based on Rick Riordan's hit young-adult series of novels.
Percy Jackson: Sea of Monsters is scheduled for release on March 27, 2013.
Watch a trailer for Percy Jackson & the Olympians: The Lightning Thief below: