The billionaire Microsoft founder tweeted that the world needs the health body more than ever.
Trump's controversial decision, which he said was made over the WHO's handling of the coronavirus pandemic, has drawn condemnation from infectious diseases experts.
It comes as the global death toll from the virus continues to mount – nearly 2 million people globally have been infected and more than 124,000 have died since it emerged in China late last year, according to a Reuters tally.
Halting funding for the World Health Organization during a world health crisis is as dangerous as it sounds. Their work is slowing the spread of COVID-19 and if that work is stopped no other organization can replace them. The world needs @WHO now more than ever.
After they were engaged, Bill and Melinda traveled to Africa in late 1993. During the trip, they encountered people in extreme poverty. On a walk on a beach in Zanzibar, Melinda told TED in an interview, the couple "started to talk about" how they might use their fortune to help others.
The couple married in 1994 in what was then Manele Bay Hotel in Hawaii. They tied the knot by the "par-3 12th hole" on the property's golf course, according to Forbes. The event reportedly cost $1 million.
Each of the Gates' kids will inherit about $10 million of their parents' $89.6 billion fortune, while much of the rest will go to charitable causes. "We want to strike a balance where they have the freedom to do anything but not a lot of money showered on them so they could go out and do nothing," Bill told TED.
Melinda has said her relationship with her husband has changed over time. "We've had to change to really be coequals," she said. "It's not something that immediately happens overnight, but we're both committed to it."
"Melinda and I believe that everyone deserves the chance for a healthy and productive life — and so with the help of our amazing partners, we are working to find innovative ways to help people in need all over the world," Bill wrote during one of his Reddit AMAs.
China has responded by urging the US to fulfil its obligations to the WHO. Foreign ministry spokesman Zhao Lijian said the pandemic was at a critical stage and that Trump's decision would affect all countries of the world.
Germany described funding of the body as one of the best investments possible. Heiko Mass, the nation's foreign minister, said on Wednesday that the "virus knows no borders".
"Apportioning blame doesn't help. The virus knows no borders," he said on Twitter. "We have to work closely together against #COVID19. One of the best investments is to strengthen the @UN, especially the under-funded @WHO, for example for developing and distributing tests and vaccines."
Australian prime minister Scott Morrison said he sympathised with Trump's criticisms of the WHO, especially its "unfathomable" support of reopening China's "wet markets," where freshly slaughtered, and live, animals are sold.
The coronavirus is believed to have emerged from such a market in the city of Wuhan late last year.
"But that said, the WHO also as an organisation does a lot of important work including here in our region in the Pacific and we work closely with them," Morrison said.
"We are not going to throw the baby out of with the bathwater here, but they are also not immune from criticism."
Gates to fund vaccine research
Gates has been vocal throughout the pandemic in calling for collaboration to beat the virus.
Earlier this month, he said the Bill & Melinda Gates Foundation would fund seven factories to help develop an effective COVID-19 vaccine as quickly as possible.
He said the foundation would help select the most promising seven potential vaccines.
"Even though we'll end up picking at most two of them, we're going to fund factories for all seven just so we don't waste time in serially saying "ok which vaccine works" and then building the factory," Gates told The Daily Show on 3 April.
"It'll be a few billion dollars we'll waste on manufacturing for the constructs that don't get picked because something else is better. But a few billion in this situation we're in, where there's trillions of dollars... being lost economically, it is worth it,"
Gates and his wife Melinda have already pledged $100 million toward fighting the pandemic.