The unscripted moment left production people shocked, though it was caught in time to censor the audio before he continued with his speech about the Boss.
He has played Special Counsel Robert Mueller on "SNL" recently in skits mocking Trump (played by Alec Baldwin). The Tony Awards crowd proved a perfectly suitable liberal environment for a fresh statement, featuring for example, the outspoken anti-Trump survivors of the Marjory Stoneman Douglas High School shooting in Parkland, Florida.
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.
But Fox News host Brian Kilmeade had a different take, saying that De Niro's comments were yet another reason not to watch "Meet the Parents", a 2000 film in which De Niro co-starred.
Trump and his top economic aides launched scathing attacks on Trudeau over the weekend amid their ongoing fight on trade. Trump advisers also ripped Trudeau, branding him a back-stabber.
De Niro was on stage at the Tonys to introduce Bruce Springsteen, whose spoken-word-into-"My Hometown" performance is a whole other conversation that you should really consider having today.
In an interview with CBC Toronto after Monday's event, De Niro said he is angry and frustrated with Trump's behavior, adding that he has "no choice but to speak out".
De Niro previously called Trump a "dog" and a "bulls*** artist" while threatening to physically assault him. What the Oscar victor had said was, "I'm going to say this: F*** Trump". De Niro said, pumping his fists in the air. "Boy, do we need that now".