Researchers say a vast area -- some 3.3 million square kilometers (1.27 million square miles) -- has been lost since the early 1990s.
The impact has been greatest in the Amazon and central Africa, they say.
The study compared recent maps of wild areas free of significant human disturbance with ones produced in the same manner in the 1990s.
James Watson from the University of Queensland
and the Wildlife Conservation Society published the results in Current Biology.
He carried out the investigation with fellow researchers.
Watson said the findings were alarming.
"The amount of wilderness loss in just two decades is staggering and very saddening," he said in a statement.
"We need to recognize that wilderness is being dramatically lost and that without proactive global interventions we could lose the last jewels in nature's crown."
The report points out that despite successful attempts to set up protected areas since the Rio Earth Summit in 1992, "the increase in protection of wilderness has lagged significantly behind losses."
Called "Catastrophic Declines in Wilderness Areas Undermine Global Environment Targets," the study says wild areas such as the Amazon are important to protect biodiversity and in the battle against climate change.
Watson called on international policy makers to tackle the problem, warning that "we probably have one to two decades to turn this around."
"If we don't act soon, it will be all gone, and this is a disaster for conservation, for climate change, and for some of the most vulnerable human communities on the planet," said Watson.
"We have a duty to act for our children and their children," he said. One of the report's authors, James Allan, said in a video post that only about 25% of wilderness was left. "Our opportunities for conservation are becoming increasingly limited," he said.
The authors warn that by 2050 there could be no wilderness left on the planet.
The report has also drawn alarm from the Wilderness Foundation
-- a charity set up to help save wild places.
In response to the study, the group's education officer Richard Corby told CNN: "In the wild, people experience a deep sense of belonging, centeredness, and peace. If humanity does not protect wild spaces, our children will never experience this transformation.
"We help people to experience wilderness because there is no better way to move them to preserve wild spaces for the future.
"Globally, the Wilderness Foundation works to protect and sustain wildlife and wilderness through integrated conservation and education programs."