If you are traveling through a Canadian airport and hold the citizenship of a country that requires a visa to visit Canada, then you will need a transit visa to travel through Canada without stopping or visiting. This is true even if you are in Canada for less than 48 hours.
There is no fee for a transit visa and you can apply for a transit visa by filling out the application for a visitor visa (Temporary Resident Visa) and selecting transit visa from the list of options on the form.
U.S. citizens do not need a visa to visit Canada and therefore, do not need to worry about a transit visa. To see if your citizenship status requires a visa, a survey is available on the Canadian government's website. Even if you don't need a visa it's possible you may need an eTA (Electronic Travel Authorization) to visit Canada. In this case, you probably will also need an eTA to transit through Canada.
What Is a Transit Visa?
A transit visa is a type of Temporary Resident Visa (TRV) required by anyone from a non-visa-exempt country who is traveling through Canada to another country and whose flight will stop in Canada for less than 48 hours. There is no cost for a transit visa, but the application process is the same as that for the TRV.
When and Where to Apply for a Transit Visa for Canada
You must apply for a transit visa for Canada at least 30 days in advance of your departure. If you are mailing it in, allow at least eight weeks for processing.
Visitors must apply for a transit visa for Canada from their country of residence. You may not apply for a visa upon your arrival in Canada. Unless told otherwise, travel agents or cruise lines will not take care of your transit visa—it is your responsibility.
How to Apply for a Transit Visa
The Temporary Resident Visa (TRV) has three types: single entry, multiple entry, and transit. To apply for any one of these types of TRV, fill out the two-page application for a Temporary Resident Visa Made Outside of Canada or call the nearest Canada Visa Office. At the top of the application, you will choose the box named "Transit." You can mail in your application to a Canada Visa Office or fill it out online, which is must faster. You will not have to include a payment as the Transit Visa is free.