Katherine Sandler is a Canadian playwright and theatre director. She is the co-founder and artistic director of Theatre Brouhaha in Toronto, where she staged a number of her plays. Her notable works include the Dora Mavor Moore Award-winning Mustard, and Yaga, which she also adapted into a television series.
Sandler was born in Toronto and grew up in the Cabbagetown neighbourhood. Her mother, Ann MacNaughton, was a screenwriter for Canadian television shows including Traders and E.N.G., and her father, Jerry Sandler, had also worked in the film industry. She attended secondary school at the University of Toronto Schools in the class of 2004. She wrote and directed her own musical as her final school project. She then studied theatre at the Dan School of Drama and Music at Queen's University, graduating in 2008.
After university, Sandler worked with Theatre Gargantua in Toronto and acted in their productions of Fibber (2008) and Imprints (2011). She considers herself to be primarily a playwright and director, and she has directed many of her own plays. In 2010, Sandler and her Queens University classmate Tom McGee founded Theatre Brouhaha, which she described as "theatre for a younger generation", with "slick, fast-paced plays that are marketed like television". The company's first production was her play Lovesexmoney in 2011.
Sandler won the New Play Contest at the Toronto Fringe Festival with Help Yourself in 2012. In 2016, she was awarded the Dora Mavor Moore Award for Outstanding New Play for Mustard. She had developed the play, about a troubled teenager's imaginary friend, at the Tarragon Theatre's Playwrights Unit.
In 2019, Sandler's play Yaga, a reimagining of the myth of Baba Yaga, debuted at the Tarragon Theatre, starring Seana McKenna in multiple roles, including a university professor with "a taste for younger men". The story blends folklore with a whodunit, following a detective investigating the disappearance of a student who interviewed the professor about Baba Yaga. Sandler adapted the play into a television series of the same name for the Canadian streaming service Crave, and she was also the showrunner for production in 2026.