Mr Brown has been campaigning tirelessly in recent months for wealthy nations and the private sector to ensure the equitable distribution of Covid-19 vaccines.
He is calling for a concerted global effort to save lives, especially in poorer countries, to help end the pandemic and restore livelihoods the world over.
Dr Tedros Adhanom Ghebreyesus, WHO director-general, said: “I am delighted and honoured that Gordon Brown has agreed to serve as WHO ambassador for global health financing.
“In this role, he will elevate and support WHO’s work to raise awareness internationally on the great need for sustained global health financing, particularly from G20 and G7 countries.
“As a longtime friend of Gordon’s, I know that he will bring his sharp intellect, firm commitment and deep sense of justice to this ambassadorship.”
Mr Brown said: “I have a tremendous personal regard for Dr Tedros with whom I first worked two decades ago, and I am delighted to be able to help his and WHO’s work raising global finance to ensure everyone has access to health.
“Our immediate task is to work together to finance the vaccination of the whole world and protect the poorest countries from the terrible effects of Covid-19 and other diseases.”