Are winter tires legally required to drive in Canada?

It depends on the province. Two provinces have legal mandates:

• Quebec: winter tires are mandatory on all passenger vehicles registered in Quebec from December 1 to March 15. The law has historically been interpreted to include rentals operating in Quebec during that window. Most Quebec-based rentals automatically equip winter tires. Out-of-province rentals operated in Quebec are technically required to comply; in practice, enforcement against rental tourists has been inconsistent.
• British Columbia: winter tires (or chains in some categories) are required on designated highways from October 1 to April 30 (until March 31 for some highways). The Sea-to-Sky Highway from Vancouver to Whistler is included. Look for the white sign with a tire-and-snowflake logo.

Elsewhere (Ontario, Alberta, etc.), winter tires are not legally required but strongly recommended.

When booking your rental: ask explicitly for winter tires if your itinerary includes Quebec in winter or BC mountain highways. Major rental companies (Enterprise, Budget, Hertz) charge a small daily premium (~CAD$5–10) for winter-tire-equipped vehicles. M+S all-season tires are not the same as proper winter tires (mountain-snowflake symbol).