"I just want to make a note of apology for the idiotic behaviour of my president", De Niro said, winning big cheers from the dozens who turned up for the groundbreaking ceremony.
The CBS television audience heard dead silence instead of De Niro's comments, The Associated Press reports.
The president called Trudeau "disgusting" and "weak" for how he pushed back against steep tariffs imposed by the US on steel and aluminum imports from Canada and other trade allies.
Robert De Niro, fresh off the Tony Awards stage where he attacked the US president, and celebrity chef Nobu Matsuhisa are in Toronto today to launch a major new development.
Trump and Trudeau have been engaged in a war of words over trade since the weekend's Group of Seven economic summit.
Trump advisers also attacked the Canadian PM, branding him a back-stabber.
When asked about De Niro's remarks, Seawright said he probably would have chosen different words but seemed to praise De Niro for voicing people's "frustration".
Trump's spate of Twitter tirades has seen a number of political leaders rally around Trudeau, including Conservative leader Andrew Scheer and incoming Ontario premier Doug Ford. He went on to argue that people should be upset about Trump's rhetoric if they were angry about De Niro's.
"America is being run by a madman who wouldn't recognize the truth if it came inside a bucket of his beloved Colonel Sanders Fried Chicken", De Niro said.
His comments were bleeped from the broadcast by CBS, which issued a statement saying: 'Mr.