Tompkins Conservation signed an agreement with Chile's government on Wednesday to donate 1 million acres for new national parks in the largest private donation of its kind for the South American nation.

Chilean President Michelle Bachelet signed the deal with Kristine McDivitt Tompkins, the widow of American conservationist Doug Tompkins, who built a legacy protecting threatened ecosystems in Argentina and Chile.

"This is a key step to treasuring this giant source of biodiversity and safe keep it in the public interest," Ms Bachelet said at a ceremony in southern Chile.

The agreement will provide land to create three new national parks, expand three existing national parks and unite some national forests into two national parks. Ms Bachelet is expected to sign the decrees to create the parks before she ends her presidential term in March 2018.