Trump justified the tariffs as necessary to manage America's opioid crisis, which he said was partially due to fentanyl being smuggled into the U.S. via Canada.
The two countries have been at the negotiating table discussing trade after Canada dropped plans for a tax on U.S. technology firms following pressure from the White House.
Trump closed the letter by suggesting if Canada assists the U.S. in preventing fentanyl from entering the country, then Trump would "perhaps" consider adjusting the tariff rates.
Earlier Thursday, a federal judge in New Hampshire said he'll certify a class action lawsuit including all children who'll be affected by President Donald Trump's executive order ending birthright citizenship and issue a preliminary injunction blocking it.