Marit Stiles (born September 20, 1969) is a Canadian politician who has been the leader of the Ontario New Democratic Party (NDP) and the leader of the Official Opposition since 2023. Stiles was elected to the Legislative Assembly of Ontario in 2018 and serves as the member of Provincial Parliament (MPP) for Davenport.

Born in Newfoundland, Stiles moved to Ontario to attend Carleton University. She worked as a policy researcher before becoming research and policy director with ACTRA. Stiles served as a Toronto District School Board (TDSB) trustee in 2014 and was the president of the New Democratic Party of Canada from 2016 to 2018.

Marit Stiles was born on September 20, 1969, in St. John's, Newfoundland, growing up in the communities of Long Pond and Logy Bay. The name Marit is of Norwegian origin, although Stiles does not have Norwegian ancestry, and the name was suggested by Norwegian family friends. Her parents were Americans who had moved from Pennsylvania to Newfoundland in 1967 and settled on a small organic farm outside St. John's, growing vegetables and raising goats, chickens, geese, rabbits and pigs. Her father was an anthropology professor at Memorial University, who later became an energy conservation consultant in Canada and Africa. She competed in synchronized swimming in her childhood, alongside her younger sister Enid.

While she first attended Memorial University, in 1988 Stiles moved to Ontario to attend university there. Her first political involvement was in 1990 when she canvassed for Evelyn Gigantes along with her future partner Jordan Berger, a childhood friend with whom she became reacquainted while attending university. She graduated with a Bachelor of Arts (BA) in political science from Carleton University in 1992. She then worked in the office of Timmins MPP Gilles Bisson. She worked for the Canadian Policy Research Networks in 1995, and as a researcher for the federal NDP's Ontario caucus from 1998 to 2004. Stiles went to work for the Alliance of Canadian Cinema, Television and Radio Artists (ACTRA) in 2005, where she was the director of research, public policy and communications.

Stiles served as a trustee for the Toronto District School Board from 2014 until 2018, for ward 9.

In 2016, Stiles was elected president of the federal NDP, defeating former NDP MPs Elaine Michaud and Djaouida Sellah. Following her election, party delegates at the 2016 convention voted to remove Tom Mulcair as leader. The party later held a leadership election in 2017, which was won by Jagmeet Singh.

Contesting Davenport for the provincial NDP in the 2018 Ontario election, Stiles ran against Liberal incumbent Cristina Martins, who faced criticism for her government's handling of the Davenport Diamond rail overpass. Stiles campaigned for the electrification of the nearby GO Transit line and ensuring all “amenities” associated with the Davenport Diamond rail bridge are completed. She defeated Martins, becoming the riding's second NDP MPP.

Stiles was the party's education critic from 2018 to 2022.