The director will make a rare appearance on screen in the Bret Easton Ellis-penned film.
Ellis tweeted an on-set photograph of Van Sant as Dr Campbell, sat opposite star James Deen.
Lindsay Lohan will also star in the film, which centres around ambition and sexual obsession among a group of 20-somethings living in Los Angeles.
Paul Schrader is directing the movie.
Van Sant's next film will be the Matt Damon and John Krasinski-starring The Promised Land, which may debut before the end of 2012.