Winslet, 33, was compelled to agree to do the narration for director Fridrik Thor Fridriksson's 'The Sunshine Boy' after she watched footage that he had sent her.
The film chronicles film producer Margret Dagmar Ericsdottir as she struggles to understand her severely autistic son Keli.
"I am delighted and honoured that Kate agreed to do the narration," Contactmusic quoted her as saying.
"Her contribution plays an instrumental role in increasing awareness for autism, which many claim is the world's fastest-growing epidemic," she added.
'The Sunshine Boy' is expected to receive rave reviews when it debuts as part of the Real to Reel section of the Toronto International Film Festival on September 12.
The flick will feature musical contributions from Icelandic stars Bjork and Sigur Ros.