An Electronic Travel Authorization (eTA) is an entry requirement for visa-exempt foreign nationals travelling to Canada by air. An eTA is electronically linked to a traveller’s passport. It is valid for up to five years or until the passport expires, whichever comes first. If you get a new passport, you need to get a new eTA.
With a valid eTA, you can travel to Canada as often as you want for short stays (normally for up to six months at a time). You do not need an eTA for travel within Canada.
An eTA doesn’t guarantee entry to Canada. When you arrive, a border services officer will ask to see your passport and other documents – for example, a U.S. Green Card. You must convince the officer that you are eligible for entry into Canada.