Stephane Dujarric, the spokesman for Guterres, confirmed that the US gave its notice.
Under conditions set when the United States entered the World Health Organisation in 1948, Washington has to give a one-year notice to pull out - and meet its remaining assessed financial obligations, Dujarric said.
"To call Trump's response to Covid chaotic and incoherent doesn't do it justice," said Senator Robert Menendez, the top Democrat on the Foreign Relations Committee, who said that Congress was notified.
"This won't protect American lives or interests - it leaves Americans sick & America alone," he said.
Trump has accused the World Health Organization of bias toward China, saying it ignored early signs of human-to-human transmission of the deadly virus.
While many public health advocates share some criticism of the WHO, they question what other powers the world body had other than to work with China, where Covid-19 was first detected late last year.
Critics say Trump is seeking to deflect criticism from his own handling of the pandemic in the United States, which has suffered by far the highest death toll of any nation.