Reading from handwritten notes during an Oval Office signing for an unrelated memorandum, Trump called Canada's actions a "disgrace" and said the Canadian measures have hurt dairy farmers in Wisconsin and New York.
"Canada, what they've done to our dairy farm workers, it's a disgrace," Trump said. "Farmers in Wisconsin and New York state are being put out of business."
Trump also lobbed criticism at Canada for what he characterized as unfair trading practices in the lumber and energy sectors.
The Canadian government protects its dairy industry through import tariffs and other measures, and dairy farmers in Canada have recently slashed some of their prices to undercut US dairy farmers.
The US dairy industry -- particularly in Wisconsin, where some dairy farmers have already felt an economic impact -- has slammed the Canadian practices as unfair and called on the Trump administration to take action.
"Our federal and state governments cannot abide by Canada's disregard for its trade commitment to the United States, and its intentional decision to pursue policies that are choking off sales of American-made milk to the detriment of US dairy farmers," said Tom Vilsack, the former secretary of agriculture and current president and CEO of the US Dairy Export Council, in a statement earlier this month.
Trump's criticism came two days after he first knocked Canada's impact on Wisconsin dairy farmers during a visit to the state on Tuesday, when he vowed the US would "stand up for our dairy farmers."
The President echoed that sentiment Thursday: "We can't let Canada or anyone else take advantage of our workers or our farmers."
Trump also took the opportunity to slam NAFTA, the free trade deal linking the US, Mexico and Canada, calling it a "disaster for our country." The free trade accord slashed a slew of tariffs but left many dairy industry tariffs in place.
He said his administration would be "reporting back sometime over the next two weeks" as to how it plans to reform the trade deal.