The 22-year-old "Kick-Ass 2" actor will replace Dakota Fanning in the film, said The Hollywood Reporter.
An adaption of Susannah Cahalan's memoir, the film will be produced by Charlize Theron.
The memoir follows attempts by doctors to diagnose what's wrong with Cahalan - eventually discovering she has a new and undiscovered autoimmune disease.
Moretz is co-starring with her "Brain on Fire" producer Theron in the adaption of Gillian Flynn's second novel, "Dark Places".