The lake was renamed after trade talks between the US and Canada collapsed last week
US President Donald Trump signed an executive order on Thursday to rename Lake Ontario as Lake America, a change he said would take effect immediately.
A White House aide said the order directs the Department of the Interior and Secretary Doug Burgum to make the change.
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.
Prime Minister Mark Carney's office did not immediately respond to a request for comment.
Trade talks between the US and Canada collapsed last week with each side blaming the other for derailing three days of intensive negotiations.