The change would take effect immediately, said Trump, who also ordered the Gulf of Mexico renamed as the Gulf of America after assuming office last year.
"We're going to be changing the name of Lake Ontario effective immediately to Lake America," Trump told reporters in the Oval Office.
"The lake change, the Lake of America, was something I've been thinking about for a long time, actually," Trump said.
Trump signed an executive order that updated the lake's name for the US geographic naming service.
The president cannot force Canada to follow his preferred naming convention, however.
Trade talks between the US and Canada collapsed last week with each side blaming the other for derailing three days of intensive negotiations.
One of the five Great Lakes of North America, Lake Ontario borders both the Canadian province of Ontario and the US. state of New York.
Canadian Prime Minister Mark Carney rejected the name change in a social media post.
He said Lake Ontario takes its name from the Wendat word "Ontari'io," meaning "the lake is beautiful, the lake is big" and said the name predates both the Canadian Confederation and the US Declaration of Independence.
"We know that America is changing. Their trading relationships, their foreign policies, their national monuments, their hydronyms," Carney said.
"Canadians also know that naming reality means calling it Lake Ontario - then, now and always."
After Trump ordered the Gulf of Mexico renamed in January 2025, it took about two weeks for the US Board on Geographic Names, as part of the Department of the Interior, to update the official federal nomenclature in the Geographic Names Information System to reflect the changes, effective immediately for federal use.
About 52 per cent of the surface area of Lake Ontario is in Canada, with the remainder in New York under a 1909 treaty.
with AP
