Kristine McDivitt Tompkins, the widow of US billionaire Doug Tompkins, is donating 408,000 hectares of land to Chile for national parks to be created.
Doug Tompkins, one of the founders of the outdoor clothing brand The North Face, died in a kayaking accident in Chile in 2015.
A keen conservationist, he bought up huge swathes of land in southern Chile and Argentina to preserve it.
His widow said Mr Tompkins' vision had inspired her to make the pledge.
"I know that if Doug were here today, he would speak of national parks being one of the greatest expressions of democracy that a country can realise, preserving the masterpieces of a nation for all of its citizenry," she said.
The donated 408,000 hectares (one million acres) will form part of a network of 17 national parks in Patagonia covering an area the size of Switzerland.
Chilean President Michelle Bachelet said that it was a "key step to treasuring this giant source of biodiversity and safe keep it in the public interest".
'High point for conservation': Gideon Long, BBC News, Santiago
The agreement between the Chilean state and the Tompkins family marks a high point for conservation in Patagonia and shows how far relations between the two have improved since Douglas Tompkins first arrived here in the early 1990s.
Back then, many in Chile regarded him and his wife Kristine with suspicion. Why were these rich "gringos" coming to southern Chile and buying up vast tracts of land? Some thought they were spies, or had plans to set up a Zionist state.
The Tompkins created their Pumalin Park in the narrowest part of what is already an extraordinarily narrow country. Chileans worried that the Tompkins would eventually own land from the coast to the Argentine border, effectively splitting their country in two.
But as the ecology movement took root in Chile, relations improved. The Tompkins have won over their sceptics with their commitment to "rewilding" Patagonia - returning it to nature and mitigating the impact of man.
The tragedy of this latest announcement is that Douglas Tompkins didn't live to see it happen.
Tompkins Conservation, the not-for-profit organisation set up by Mr Tompkins and his wife, described the pledge as "the largest land donation in history from a private entity to a country".
The Chilean state agreed to add a further 949,000 hectares of land.
The 17 parks will stretch from the Chilean city of Puerto Montt down to Cape Horn, some 2,000km (1,250 miles) to the south.