More than 100 world leaders will promise to end and reverse deforestation by 2030, in the COP26 climate summit's first major deal.
Brazil - where stretches of the Amazon rainforest have been cut down - will be among the signatories on Tuesday.
The pledge includes almost £14bn ($19.2bn) of public and private funds.
Experts welcomed the move, but warned a previous deal in 2014 had "failed to slow deforestation at all" and commitments needed to be delivered on.
Felling trees contributes to climate change because it depletes forests that absorb vast amounts of the warming gas CO2.
UK Prime Minister Boris Johnson, who is hosting the global meeting in Glasgow, will call Tuesday's deal a "landmark agreement to protect and restore the earth's forests".
"These great teeming ecosystems - these cathedrals of nature - are the lungs of our planet," he will say.
The two-week summit in Glasgow is seen as crucial if climate change is to be brought under control.
The countries who say they will sign the pledge - including Canada, Brazil, Russia, China, Indonesia and the Democratic Republic of the Congo - cover around 85% of the world's forests.
Some of the funding will go to developing countries to restore damaged land, tackle wildfires and support indigenous communities.
Governments of 28 countries will also commit to remove deforestation from the global trade of food and other agricultural products such as palm oil, soya and cocoa.
These industries drive forest loss by cutting down trees to make space for animals to graze or crops to grow.
More than 30 of the world's biggest companies will commit to end investment in activities linked to deforestation.
And a £1.1bn fund will be established to protect the world's second largest tropical rainforest - in the Congo Basin.
