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.
Prof Simon Lewis, an expert on climate and forests at University College London, said: “It is good news to have a political commitment to end deforestation from so many countries, and significant funding to move forward on that journey.”
But he told the BBC the world “has been here before” with a declaration in 2014 in New York “which failed to slow deforestation at all”.
He added that this new deal did not tackle growing demand for products such as meat grown on rainforest land – which would require high levels of meat consumption in countries like the US and UK to be addressed.
Ana Yang, executive director at Chatham House Sustainability Accelerator, who co-wrote the report Rethinking the Brazilian Amazon, said: “This deal involves more countries, more players and more money. But the devil is in the detail which we still need to see.
“This is a really important step at COP26. This meeting is around increasing the level of ambition and keeping global temperature rises below 1.5C – this is a big building block,” she added. (Courtesy BBC)