President Trump signed an executive order on Thursday to change the name of Lake Ontario to Lake America, carrying out his threat to rename the easternmost Great Lake as part of an escalating trade war with Canada.

During remarks in the Oval Office, Mr. Trump said that the order, which directs the Interior Department to update the government’s Geographic Names Information System to reflect the name change, was effective immediately.

After signing the order, Mr. Trump reiterated his criticism that Canada had been taking advantage of the United States, and that the country “treats us very badly.” He added that Canada was the “worst” to deal with on trade, and he reissued his threat to impose steep tariffs on automobiles made there.

“Canada has been ripping us off for a long time on trade, very sadly,” he said. “Even the military, you know, we defend Canada for nothing. We don’t get anything.”

Canada, like the United States, is part of NATO, a security alliance with a mutual defense agreement. Canada is also one of the United States’ closest intelligence partners.

Like Mr. Trump’s order to rename the Gulf of Mexico to the Gulf of America, Thursday’s order most likely requires only federal entities to recognize the change, but it does not have to be recognized by other countries. Mr. Trump told reporters that the administration had “filed all the necessary papers, documents” and “notified all of the various people that we have to notify” to make the change.

“So we’ve done everything that you have to do,” he said, “and this is official, effective immediately.”

Canadian and American officials have both rebuffed Mr. Trump’s attempt to rename the body of water.

New York Governor Kathy Hochul dismissed Mr. Trump’s threat earlier this week as “silly talk.” And on Thursday, she reposted the White House’s new directive on social media, and said the state, which borders the lake, will not comply. “New York won’t be calling it that,” she wrote.

Wab Kinew, the premier of Manitoba, likened Mr. Trump’s actions to a has-been rock group replaying songs from their heyday.

“He thought he had a real hit, a real golden oldie, with the Gulf of Mexico, and he’s trying to bring that back,” Mr. Kinew told reporters on Thursday afternoon. “He’s trying to play the hits for his base, but I think everybody here thinks it’s a little sad, and it’s not his best work.”

Prime Minister Mark Carney also weighed in, saying in a social media post: “We know that America is changing. Their trading relationships, their foreign policies, their national monuments, their hydronyms. Canadians also know that naming reality means calling it Lake Ontario — then, now and always.”

Mr. Trump first announced in a social media post earlier this week that the United States was giving “serious consideration” to changing the name of Lake Ontario after trade talks between the two countries fell apart.

Canadian officials walked away from what they publicly described as deal-breaking demands from the United States. In the social media post, Mr. Trump said that the name change was being considered because “we don’t expect to be doing much business with Ontario any longer.”

Mr. Trump then continued to troll Canada on Wednesday by posting a photo of a map with Lake Ontario crossed out in red ink, and a video of him swiping through the name with a pen and writing “Lake America” in its place.

On Thursday, Mr. Trump displayed a large poster board depicting the new “Lake America” that blared “Making the Great Lakes Even Greater.”

Vjosa Isai contributed from Toronto.