William Sterling Blair (born 1954) is a Canadian diplomat, politician, and former police officer who has served as the Canadian High Commissioner to the United Kingdom since 2026. A member of the Liberal Party, he formerly represented Scarborough Southwest in the House of Commons from 2015 to 2026. Blair held Cabinet portfolios during the Justin Trudeau and Mark Carney governments, including the role of Minister of National Defence from 2023 to 2025. Before entering politics, Blair worked for four decades with the Toronto Police Service (TPS), serving as the chief of police from 2005 until retiring in 2015.

Blair was born in 1954 in Scarborough, Ontario. Blair's father had served as a police officer for 39 years. Blair considered pursuing a degree in law or finance when he initially enrolled at the Scarborough campus of the University of Toronto in the mid 1970s. Blair initially studied economics in university before he left to follow his ambition of being a police officer. He later returned to the University of Toronto and completed a Bachelor of Arts in economics and criminology.

Blair is married to Susanne McMaster, and together they have three grown children (2 sons and daughter) and 2 grandchildren.

Blair joined the Metropolitan Toronto Police while in university to make money and began taking courses on a part-time basis. Blair walked a beat near Regent Park and later worked as an undercover officer in Toronto's drug squad.

After Blair earned his bachelor's degree in criminology, he advanced his career in the police service in the late 1980s, taking part in drug busts involving the seizure of millions of dollars of cocaine. Chief David Boothby assigned Blair to improve the poor community relations between the officers of 51 Division, which patrolled Blair's old beat near Regent Park. Blair normalized police relations with the community by measures such as sending cops to read to kids in local elementary schools and engaging with local businesses and churches.

In 1999, Blair was considered as a candidate to replace outgoing Chief Boothby, but Mayor Mel Lastman, with the support of Premier Mike Harris, chose to hire Julian Fantino, then head of the York Regional Police. After reorganization of the senior ranks after Fantino's ascension as police chief, Blair became head of detective operations.

Blair was selected in a 4–2 vote of the Toronto Police Services Board in early April 2005, and formally appointed Chief of the Toronto Police Service on April 26, 2005. He succeeded Mike Boyd, who had served as interim chief after the expiry of Julian Fantino's contract. Prior to his appointment as chief, Blair worked for approximately 30 years as a Toronto police officer, with assignments involving drug enforcement, organized crime and major criminal investigations. Blair served as president of the Canadian Association of Chiefs of Police.

In the spring of 2009, Tamil Canadians in Toronto upset by civilian deaths in the Sri Lankan Civil War, which included an overnight artillery bombardment that killed 378 civilians and wounded 1,100, allegedly perpetrated by the Sri Lankan military, staged a series of protests in Toronto, including shutting down the northbound and southbound lanes of University Avenue for four days while protesting in front of the US Consulate, and illegally blocking traffic on the Gardiner Expressway. Blair and the police faced pressure to crack down on the demonstrations, arrest and deporting the protesters. Instead, Blair used his experience in community policing to ensure that minimal force was used, spoke respectfully of the protesters' rights to expression, and negotiated the peaceful resolution of the events. The Canadian Tamil Congress would later award Blair an inaugural "Leaders for Change Award" for his leadership during the protests.