The Black Swan star dons a nerdy pair of spectacles and looks downtrodden for her role in independent flick, Hesher.
Portman, 29, plays Nicole, whose life is turned upside down when she meets the title character, portrayed by Joseph Gordon-Levitt.
He is almost unrecognizable as a long-haired dropout who becomes a mentor to a young boy whose mother has died.
The film is out in the US in April 2011.
Portman, who is expecting her first child, has been tipped to win an Oscar next week for her performance in ballet drama, Black Swan.