After boasting about his “truly amazing” showdown with North Korean leader Kim Jong Un in a series of tweets oboard Air Force One, the US president wasted no time on his way back to the States to punch back at the Hollywood heavyweight.
He tweeted: “Robert De Niro, a very Low IQ individual, has received too many shots to the head by real boxers in movies.
“I watched him last night and truly believe he may be ‘punch-drunk’.”
De Niro, who won the Academy Award for best actor for his portrayal of boxer Jake LaMotta in the 1980 film Raging Bull, got a standing ovation at Sunday’s event with his condemnation of Mr Trump.
Punching his fists in the air, he said: “I’m going to say one thing – f*** Trump!
“It’s no longer ‘down with Trump’, it’s ‘f*** Trump!'” he told the crowd at New York’s Radio City Music Hall honouring the best performances on Broadway.
The CBS television network, which was broadcasting the event live but with a delay of a few seconds, bleeped out De Niro’s expletive.
Mr Trump defended himself against De Niro’s attack by saying the US economy was booming under his presidency.
He wrote: “I guess (De Niro) doesn’t realise 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. Wake up Punchy!”
De Niro has previously described the president as “blatantly stupid,” “totally nuts” and an “idiot”.
He also threatened to “punch him in the face” in the run-up to the 2016 election.