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Updated Nov. 25, 2024 at 8:20 pm ET

Trucks wait in a queue to cross into the U.S. from Mexico. Photo: Daniel Becerril/Reuters

WASHINGTON—President-elect Donald Trump pledged that soon after taking office he will slap steep tariffs on Mexico and Canada, two of America’s closest allies, as well as China, the clearest indication since his election victory that he plans to follow through on the tough campaign rhetoric that helped propel him to the White House.

Posting on his Truth Social platform, Trump said that on the first day of his presidency he will charge Mexico and Canada a 25% tariff on all products coming into the U.S. He added in a separate social-media post that he would impose an additional 10% tariff on all products that come into the U.S. from China, though he didn’t specify that additional levy would come on his first day in office. That would come on top of existing tariffs the U.S. has already imposed on Chinese goods.

“This Tariff will remain in effect until such time as Drugs, in particular Fentanyl, and all Illegal Aliens stop this Invasion of our Country!” Trump wrote, referencing his proposed tariffs on Canada and Mexico.

Doing so would likely upend the trade agreement that Trump negotiated with the two neighboring countries in his first term, known as USMCA. U.S. goods and services traded utilizing that trade agreement totaled an estimated $1.8 trillion in 2022, according to the office of the U.S. Trade Representative, which oversees U.S. trade agreements.

Economists have warned that Trump’s plan to impose sweeping tariffs could lead to higher prices on goods for consumers.

Trump faulted the two border countries and China for, in his view, not doing enough to stem the flow of drugs or migrants into the U.S. He said Beijing had not made good on its promise to prevent fentanyl from coming into the country.

“Until such time as they stop, we will be charging China an additional 10% Tariff, above any additional Tariffs, on all of their many products coming into the United States of America,” the president-elect wrote, referring to China.

Representatives at the embassies for China, Mexico and Canada didn’t immediately respond to a request for comment.

Mexico, China and Canada are the nation’s top three suppliers of imported goods, accounting for about 42% of imports to the U.S. this year through September, according to census data. Canada and Mexico send about 80% of their exports to the U.S., and an across-the-board tariff would hurt their economies.

Trade was at the center of Trump’s successful presidential bid. In campaign speeches, he said he would put in place an across-the-board tariff of up to 20% on U.S. imports.

Billionaire Elon Musk, who is advising Trump, posted on X that the proposed tariffs “will be highly effective” in stopping illegal migration and drug trafficking.

A caravan of about 1,600 migrants, most of them from Venezuela and Central America, left the southern city of Tapachula near Mexico’s border with Guatemala on Wednesday. U.S.-bound migrants said that they decided to march north and reach the border before Trump takes office in January. Trump’s tariff threat comes as human smugglers across the region are urging migrants on social media to rush to the border before the inauguration.

So far, six caravans with thousands of migrants have departed from southern Mexico since President Claudia Sheinbaum took office last month. But over the past five years, Mexican authorities have disbanded all caravans before they reach central Mexico.

In recent months, authorities have erected security checkpoints and thousands of asylum seekers caught heading north have been put on buses and sent back to southern Mexico under a enforcement strategy implemented after large migrant groups rushed to border communities such as Eagle Pass, Texas, in late 2023.

Sheinbaum has said that Mexico’s enforcement measures have helped reduce illegal crossings at the U.S.-Mexico border by 74% since December. “The model is working, and we want it to continue working,” she said earlier this month.

Canada, the world’s fourth-largest oil producer, sends most of its oil to the U.S. It also has an auto industry that is tightly integrated with its American neighbor. Economists at the Business Development Bank of Canada said the Canadian economy would suffer steep drops in output and employment if Trump imposed tariffs on Canadian imports.

Trump has said he wants to renegotiate the USMCA under provisions of the trade deal allowing for a review of the agreement in 2026. Canada has said it is deploying a team of Canadian government ministers and provincial leaders to court lawmakers in U.S. states and Washington as part of a lobbying effort to protect its $900 billion trade relationship with the U.S.

During the presidential campaign, Nikki Haley and Vivek Ramaswamy called for more attention to the northern border, with Ramaswamy at one point proposing to build a wall along the frontier with Canada.

Anthony DeBarros, Santiago Perez and Vipal Monga contributed to this article.

Write to Natalie Andrews at natalie.andrews@wsj.com and Andrew Restuccia at andrew.restuccia@wsj.com