Trump has wide latitude over how the US government recognises geographic places and landmarks but he cannot force Canada or individual citizens on either side of the border to follow his preferred naming conventions.
And his latest branding idea - reminiscent of his move to rechristen the Gulf of Mexico as the "Gulf of America" - highlights how much his rhetoric and trade policy have sundered the previously warm relationship between Washington and Ottawa.
Trump introduced the idea of renaming the easternmost area of the Great Lakes hours before Canadian Prime Minister Mark Carney's scheduled announcement of reciprocal tariffs on US imports into his country.
"The United States is giving serious consideration to changing the name of Lake Ontario to Lake America in that we don't expect to doing much business with Ontario any longer," Trump posted on Truth Social.
The White House followed later in the day by posting on social media a toy Lego map of the US labelling the Gulf of Mexico with Trump's preferred name, Gulf of America.
The US and Canada, which share borders along multiple Great Lakes, including Lake Ontario, are locked in an extended trade dispute, with the Trump administration imposing 50 per cent tariffs on $US20 billion ($A28 billion) worth of Canadian goods over the weekend after talks between the countries broke down.
That ratcheted up lower tariffs that Trump imposed in 2025, shortly after his second presidency began.
Trump has also threatened new 50 per cent tariffs on Canadian vehicles, auto parts and steel, while Carney said US trade demands showed that Washington wanted to "destroy our major industries," including autos, steel and aluminium.
The official US-Canada border stretches across the water between Lake Ontario's northern and southern shores.
The body of water forms much of New York State's coastline and separates Buffalo from Toronto, the capital of the province of Ontario. Toronto is Canada's most populous city and Ontario its most populous province.
Ontario Premier Doug Ford, who traded personal insults with Trump on Monday, dismissed the Lake Ontario threat as "a lot of rhetoric" and urged the two countries to resume negotiations.
Ford acknowledged in a CNN interview that the dispute had "got a little personal" and agreed it was time to lower the temperature, saying the trade fight was hurting both countries and that Canada and the United States should "get back to the table" to negotiate.
The lake's name comes from the Huron Indigenous people's word "oniatarí:io," which means "lake of shining waters" and it predates European settlement in the region. The province, founded in 1867, took its name from the lake.
