"Well, Kim Jong Un made a statement that Joe Biden is a low IQ individual. He probably is, based on his record. I think I agree with him on that," Trump told a news conference in Tokyo.
Trump's comments on the world stage reinforced a tweet that he sent on Saturday with a similar message and drew renewed criticism of the president back home.
North Korea's state-run news agency issued a stinging attack last week on Biden, who has been critical of the isolated state.
Trump has targeted Biden with increasing criticism as the former vice president rises in polls for the Democratic presidential nomination to challenge the Republican Trump in the November 2020 election campaign.
The president often derides his political opponents, but his tweet on Sunday and his comments on Monday were notable because he issued them while abroad, a practice traditionally shunned in U.S. politics, and aligned himself with Kim, considered a brutal dictator by many.
Trump spoke during the U.S. Memorial Day weekend, a federal holiday honoring Americans who have died while serving in the armed forces.
"Backing ruthless dictators over our allies in the region?" tweeted U.S. Senator Amy Klobuchar, a Democratic presidential hopeful. "This does not show American strength: Still angling for a deal, Trump backs Kim Jong Un over Biden, Bolton and Japan."
Trump's national security adviser, John Bolton, last week said North Korean missile tests had "no doubt" violated U.N. Security Council resolutions. On Monday, Trump alluded to Bolton and said he had a different view.
'DISGRACEFUL'
"We're going to have to retire the word 'unprecedented' during Trump's presidency," said Larry Sabato, presidential historian and director of the University of Virginia's Center for Politics.
"It's becoming the new normal. And disgraceful, of course," he wrote on Twitter.
One of Trump's fellow Republicans, US Representative Adam Kinzinger, called out the president on Sunday for "praising a dictator" and attacking Biden over the Memorial Day weekend.
"It's just plain wrong," Kinzinger, a military veteran, said on Twitter.
Biden's campaign and the Democratic National Committee did not immediately respond to requests for comment.
Pete Buttigieg, a Democratic candidate for president and Navy veteran, was also critical on Monday. "Kim Jong Un is a murderous dictator and Vice President Biden served this country honorably. It's just one more example, though, of the way that this president tries to draw attention to himself by saying things that shock the conscience ..."
Trump has sought to build a strong relationship with the North Korean leader in the hope that North Korea would agree to denuclearize.
Trump also knocked Biden and former President Barack Obama on Monday for their efforts to prevent Iran from getting a nuclear weapon. Trump pulled the United States out of a nuclear deal Iran sealed with Washington and other foreign powers.
"Joe Biden was a disaster, his administration with President Obama, they were basically a disaster when it came to so many things, whether it was economy, whether it was military ... no matter what it was, they had a lot of problems," Trump said when asked about criticism that he was favoring Kim over the former vice president.
"So, I'm not a fan."
A commentary by North Korea's KCNA state media on Tuesday slammed Biden for "rhetoric slandering the supreme leadership of the DPRK (Democratic People's Republic of Korea)."
"What he uttered is just sophism of an imbecile bereft of elementary quality as a human being, let alone a politician," the North Korean news agency said.