The discovery was made in Alatash National Park in northwest Ethiopia, on the border with Sudan, the Britain-based Born Free Foundation said in a statement.
Conservationists obtained camera trap images of lions and also identified lion tracks, confirming reports from local residents that lions were in the area, it said.
Born Free funded the research, which was carried out by a team led by Hans Bauer. He works for the Wildlife Conservation Research Unit at Oxford University in Britain. Ethiopian conservationists were also involved.
It said the park has never been visited by tourists and that its tourism potential is low, "mainly due to remoteness, climate, occasional insecurity and low probability of observing flagship species of wildlife."
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Africa's lion population has declined from up to 500,000 early in the 1900s, down to less than 200,000 by the middle of the century, and now as few as 20,000 remaining in the wild, according to estimates.
The welfare of lions came under increased scrutiny last year when an American dentist killed a well-known lion named Cecil in Zimbabwe in what authorities there said was an illegal hunt.
The case prompted an international outcry and renewed debate about the ethics of hunting threatened species.
A few rare Abyssinian lions are still kept at the Ethiopian national palace in the capital, Addis Ababa, and US President Barack Obama was taken to visit them during a trip to Ethiopia last year.