Citizens of certain countries and territories need a visa to enter Canada. If you are among them, you will need a transit visa to travel through Canada without stopping or visiting. This applies even if you are in Canada for less than 48 hours.
You may not need a transit visa if you are travelling to or from the United States. The Transit Without Visa program (TWOV) and the China Transit Program (CTP) allow certain foreign nationals to transit through Canada on their way to and from the United States without a Canadian transit visa, if they meet certain requirements.
To learn more, visit the website of Government of Canada