The UN warned that failure to act could undo decades of development. It initially asked for $2 billion in its first coronavirus appeal in March.
The coronavirus pandemic is having a huge impact on the world's poorest, the BBC's Imogen Foulkes reports from Geneva.
This revised appeal is a record, but, the UN says, wealthy countries have thrown away the financial rule book to protect their own economies, and must now do the same for poorer nations.
If they do not, the UN warns, the world faces a series of crises, with millions pushed into starvation.
Millions of migrant workers laid off under lockdown cannot send money home, vaccination programmes for childhood diseases are on hold, and countries already enduring years of conflict are ill equipped to handle Covid-19.
In Yemen, a quarter of all those confirmed to have had the virus have died from it, five times the global average.
It comes as an appeal to help the world's most vulnerable through the pandemic was launched by the UK's Disasters Emergency Committee (DEC).
Fourteen charities - including Oxfam, Christian Aid, Islamic Relief and the British Red Cross - will join together to ask the British public to donate.
There have been more than 13 million confirmed Covid-19 cases so far globally and nearly 600,000 people have died.