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.

Marielle Stiles Heller is an American director, screenwriter and actress. She is best known for directing the films The Diary of a Teenage Girl (2015), Can You Ever Forgive Me? (2018), and A Beautiful Day in the Neighborhood (2019), as well as acting in The Queen’s Gambit (2020).