Douglas Tompkins, who died in December, was one of the founders of the outdoor clothing brand The North Face.
His widow, Kristine McDivitt Tompkins, said she had formally offered 400,000 hectares of land in the Patagonia region to be made into national parks.
The negotiations are expected to take two years.
A globe-trotting rock climber and skier in his youth, Douglas Tompkins got divorced in the 1990s, abandoned corporate life and moved to isolated southern Chile.
He caused controversy in the 1990s by buying up land in southern Chile and Argentina to preserve it.
Profile: Douglas Tompkins, billionaire anti-capitalist
With his second wife Kristine, a former CEO of outdoor clothing company Patagonia Inc, he began buying up land in order to protect an ancient forest in Patagonia.
Over 25 years they invested more than $375m in conservation, donating part of their lands in Chile and Argentina to create four national parks.
Mr Tompkins was killed in a kayaking accident last year.
Kristine McDivitt Tompkins discussed the donation with Chile's President Michelle Bachelet.
Last month, she met the new Argentinean President Mauricio Macri to donate 150,000 hectares of threatened wetlands near the border with Brazil, with the aim of creating the Ibera National Park.
After the meeting with President Bachelet, Mrs Tompkins said that the donation of the latest tranche of land to Chile was being made on condition that it would be used to create national parks which people could freely visit.
"The process will take a long time," she said, " We won't be deciding the timing and I know there will be a compromise both sides will have to make to work through all the stages that will be necessary.
"We want people from all over the world to be able to visit these places. With the donation of these parks, Chile will be able offer a historic legacy to the world."
In 2005, the Tompkinses donated another tranche of land to Chile, 294 hectares near the Corcovado volcano, creating the sixth largest national park in Chile - the Corcovado National Park.
Talks have begun between Chile and the widow of an American billionaire over the donation of a vast nature reserve in southern Chile.