As he spoke, he raised his fists into the air, taking in the shock, awe, and admiration from the audience at Radio City Music Hall. He delivered his response on his Twitter account while on his way back to the United States.
What did De Niro say exactly that would take away from the attention the president's historic summit received?
Trump went on to write: "I guess he doesn't realize the economy is the best it's ever been with employment being at an all time high, and many companies pouring back into our country".
With the audience of theater actors, directors and producers shrieking and rising to their feet in applause, De Niro said: "It's no longer down with Trump".
"I'm going to probably have to move there", De Niro joked on the day of the result.
This is not the first time that De Niro has made his thoughts on Trump known.
De Niro said Trump's remarks were a "disgrace". (Pictured at the 2005 Tribeca Film Festival).
De Niro has attacked President Trump on previous occasions, describing him in 2016 as "blatantly stupid", "totally nuts" and an "idiot".
"I just want to make a note of apology for the idiotic behavior of my president", De Niro said in Toronto on Monday for the groundbreaking of the Canadian Nobu restaurant, where he was dining with celebrity chef and business partner Nobu Matsuhisa, The Globe and Mail reported. And I apologize to [Canadian Prime Minister] Justin Trudeau and the other people at the G7.