He has snapped up more than half a million acres of the Esteros del Ibera, a vast Argentine marshland teeming with wildlife.
Tompkins, 64, is a hero to some for his environmental stewardship. Others resent his land purchases as a foreign challenge to their national patrimony.
In an interview with The Associated Press, Tompkins said industrialized agriculture is chewing up big chunks of Argentina's fragile marshland and savanna, and that essential topsoil is disappearing as a result.
"Everywhere I look here in Argentina I see massive abuse of the soil ... just like what happened in the U.S. 20 or 30 years ago," he said.
Tompkins hopes to do in Argentina what he did in Chile — create broad stretches of land protected from agribusiness or industrial development, and one day turn them over to the government as nature reserves.
Wealthy foreigners have bought an estimated 4.5 million acres in Argentina and Chile in the past 15 years for private Patagonian playgrounds. Sylvester Stallone, Ted Turner and Italian fashion designer Luciano Benetton all have large holdings set amid pristine mountains and lakes.
Tompkins was among the early ones, buying a 35-mile-wide strip of Chile from a Pacific coastal bay to the country's Andean mountain border with Argentina. He said his purchases were intended specifically to protect the environment.
Argentine officials took notice in the late 1990s and aggressively courted Tompkins' philanthropy, flying him to several areas of ecological significance.
"The land conservation budget was burning a hole in our pocket," Tompkins said.
He bought a 120,000-acre ranch in 1998 and has increased his Argentine holdings to nearly 600,000 acres since then. He now owns well over 1 million acres in Chile and Argentina, a combined area about the size of Rhode Island.
Critics weave conspiracy theories, accusing Tompkins of seeking control of one of South America's biggest fresh water reserves, and worrying that he might never cede the lands to the state.
"These lands should not belong to an individual, much less a foreigner," said Luis D'Elia, who argues the American could gain "control of resources that are going to be scarce in the future, like water."
Tompkins' Argentine holdings sit atop the huge Guarani Aquifer, which extends north into Paraguay.