President Trump wasted no time after tying a bow on his meeting with Kim Jong-un to finally punch back, so to speak, at the actor Robert De Niro, who spent the last couple of days publicly slamming the president, going as far as to use profanity during the Tony Awards on Sunday night.

“____ Trump,” Mr. De Niro shouted into the microphone on Sunday, before introducing Bruce Springsteen. “It’s no longer ‘Down with Trump!’ It’s just ‘____ Trump!’” he said. The crowd gave him a standing ovation. Mr. De Niro flexed both of his arms.

At an event in Toronto on Monday, Mr. De Niro apologized to Prime Minister Justin Trudeau of Canada, and to others who attended the Group of 7 summit meeting in Quebec last week, for what he called the president’s “idiotic behavior.”

President Trump had refused to sign a communiqué at the summit, and called Mr. Trudeau “dishonest” and “weak” in a tweet afterward. Mr. De Niro said Mr. Trump’s behavior was “a disgrace” and “disgusting.”

In a two-tweet thread on Tuesday afternoon, Mr. Trump called Mr. De Niro “a very Low IQ individual,” who had “received to many shots to the head by real boxers in movies.”

“I watched him last night and truly believe he may be ‘punch-drunk,’” added the president, who was en route back to the United States from Singapore. It was unclear if he was referring to Mr. De Niro’s remarks at the Tonys or in Canada.

“Wake up Punchy!” the president concluded, after boasting about the United States economy.

Representatives for Mr. De Niro declined to comment on the president’s tweets.

About 15 minutes before posting about the actor, the president tweeted that he had gotten along “great with Kim Jong-un,” the North Korean leader. “Great progress was made on the denuclearization,” the president wrote.

Sunday night was not the first time Mr. De Niro, who starred in one of Mr. Trump’s favorite movies, “Goodfellas,” had used a public stage to hurl insults and expletives at the president. In January, before introducing Meryl Streep at the National Board of Review awards gala, he used profanities when referring to Mr. Trump. Mr. De Niro also called Mr. Trump a “fool” and the “baby-in-chief.”

And his tweet on Tuesday was not the first time Mr. Trump had called someone a “low IQ individual.” He used that same insult to describe Mika Brzezinski, the co-host of “Morning Joe” on MSNBC, in June of last year, and Maxine Waters, a Democratic congresswoman from California, in March.

Sopan Deb contributed reporting.