Born Free Foundation said it had obtained camera trap images and identified lion tracks in the Alatash area close to the border with Sudan.
The area is thought to have lost all its lions in the 20th Century because of hunting and habitat destruction.
The number of lions in Africa has declined by half since the 1990s.
The lions are thought to be of the Central African sub-species, of which only about 900 were thought to survive, Born Free Foundation's programmes manager, Mark Jones, told the BBC Newsday programme.
"Even though the team only visited the Ethiopian side of the park because of logistics, lions were likely to exist in the larger, adjacent Dinder National Park across the border in Sudan," he said.
Dr Hans Bauer, a renowned lion conservationist from Oxford University, who led the expedition, said this was the first time that the presence of lions had been confirmed in this area.
He estimated that the area could host a population of 1-200 lions.
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