'' The Diary of a Teenage Girl '' is a 2015 American comedy-drama film written and directed by Marielle Heller , based on the hybrid novel of the same name by Phoebe Gloeckner . It stars Bel Powley as a 15-year-old girl who becomes sexually active by starting a relationship with her mother 's boyfriend . It also stars Kristen Wiig , Alexander Skarsgård , Christopher Meloni , Quinn Nagle , and Austin Lyon . It premiered at the 2015 Sundance Film Festival and had a limited release on August 7 , 2015 by Sony Pictures Classics
2015 Sundance Film Festival
2015 Sundance Film Festival
The Diary of a Teenage Girl