"I just want to make a note of apology for the idiotic behavior of my president," he said. "It's a disgrace, and I apologize to [Canadian Prime Minister] Justin Trudeau, too, and the other people at the G-7."
Trump and Trudeau have been engaged in a war of words over trade since the weekend's Group of Seven economic summit. There, Trudeau announced retaliatory measures in response to Trump's tariffs on Canadian steel and aluminum.
Trudeau said Canada "will not be pushed around."
Trump responded on Twitter by slamming Trudeau as "very dishonest and weak."
De Niro's apology to Canada over the spat came one day after his profanity-laced tirade against Trump earned him a standing ovation at the Tony Awards.
The "Taxi Driver" and "Raging Bull" actor was in Canada for a groundbreaking for a new complex that will feature a Nobu restaurant, hotel and condos, according to the CBC.
De Niro, part owner of the Nobu Hotels chain, told the news organization after the event that he felt he had no choice but to speak out against the president.
"This guy just is a jerk," De Niro said.