“Congratulations to Kim Jung Un!” the former president wrote on Truth Social, misspelling his name while sharing news of the admission.
The post drew immediate condemnation from GOP presidential candidates and state party leaders - including his top 2024 rival Ron DeSantis.
Asked about the post at an Iowa campaign event on Saturday, the Florida governor said: “I was surprised to see that. I mean, one, Kim Jong-un is a murderous dictator.”
Mr DeSantis noted that North Korea recently sentenced a family with a young infant to life in prison - before shifting his condemnation to the WHO.
“The World Health Organisation is a bankrupt organisation,” he said. “Like, Kim Jong-un is bad but joining that, we need to be getting out of that, and rejecting the WHO lockdown treaty and not congratulating about being involved in the WHO.”
Nikki Haley, Mr Trump’s former UN ambassador and another 2024 opponent, also rebuked Mr Trump.
“Kim Jong Un starves his own people,” Ms Haley said on Twitter.
Desantis criticizes Trump for his post congratulating “murderous dictator” Kim Jong-Un. pic.twitter.com/89pFERRpif
— Ron Filipkowski (@RonFilipkowski) June 3, 2023
“It’s a total farce that North Korea has a leading role at the World Health Organisation.”
Georgia Governor Brian Kemp wrote: “Taking our country back from Joe Biden does not start with congratulating North Korea’s murderous dictator.”
Former GOP Maryland governor Larry Page also derided Mr Trump: “Kim Jong-un is an enemy of America who threatens peace and freedom. The fact that Trump delusionally believes otherwise makes him a useful idiot for China and unfit to be president.”
Mr Trump famously enjoyed a warm relationship with Mr Kim during his one-term presidency, frequently praising the 39-year-old despot and becoming the first sitting US leader to visit the isolated communist country.
He even bragged about exchanging “love letters” with Mr Kim, who frequently threatens the US and its allies with nuclear attack.
Others pointed out the peculiarity of their relationship.
“Fanboying a murderous dictator while misspelling his name is very on-brand for him,” actor Mark Hamill wrote on Twitter.
Dr Jong Min Pak this week became the first North Korean official to be appointed for a three-year term to the WHO’s executive board.
“What this means is that one of the world’s most horrific regimes is now a part of a group that sets and enforces the standards and norms for the global governance of health care,” UN Watch executive director Hillel Neuer said in a statement.
“It is an absurd episode for a key UN agency that is in much need of self-reflection and reform.”
Others to join the executive board are Australia, Barbados, Cameroon, Comoros, Lesotho, Qatar, Switzerland, Togo and Ukraine.
Several recent appointments of serial human rights abusers to leadership roles in international organisations have also faced condemnation.
Russia took charge of the United Nations Security Council in April, while Iran is chairing the UN Human Rights Council 2023 Social Forum.
A South Korean lawmaker this week said that Mr Kim was obese and suffering from severe insomnia and alcohol addiction.