Bottom Line: Travelers from all countries, including the United States, must show a valid passport or equivalent travel document to enter Canada by air.
Visitors from certain countries also require a visa to visit or travel through Canada.
As of September 2016, visitors who previously did not need a visa, have needed to show an Electronic Travel Authorization.
Best Advice: If you are still unsure about the passport requirements for your flight to Canada, consult the airline with which you are traveling. Passengers must comply with the airline's regulations, which are spelled out on their websites.
If you need a passport right away, get a passport within 24 hours with Rushmypassport.com
In Depth: Although the Canada Border Services Agency does not technically require U.S. citizens to have an American passport to enter Canada by air, the Western Hemisphere Travel Initiative (WHTI) — a U.S. plan introduced in 2004 to increase border security — does require U.S. citizens to present their passports upon re-entry. In this way, the U.S. and Canada border laws may be different on paper, but, in practice, they are the same because the airlines will require passengers to show a valid passport or other valid travel document: no sense going somewhere if you can't return home again.
For more information, visit Canada Border Services Agency or U.S. Department of State.
Further Reading about the Canadian Border:
Tips for Driving across the Border into Canada
Duty Free Shopping at the Canadian Border
Bringing Children across the Canadian Border
How Much Liquor Can I Buy at the Border?