One of the most secretive large mammals on Earth, the pygmy hippo is rarely seen in the wild but was caught on camera in the west African country's only national park, Sapo, by a team led by the Zoological Society of London (ZSL).
The hippo, which looks like a smaller version of the common hippopotamus, had been identified by ZSL's Evolutionarily Distinct and Globally Endangered (Edge) programme as being in need of urgent conservation, prompting the trip to Liberia.
There are thought to be less than 3,000 of the mammals remaining in the wild in Sierra Leone, Ivory Coast, Guinea and Liberia and there are serious concerns about its survival due to habitat destruction and poaching.
But "camera traps" set up by ZSL, Flora and Fauna International (FFI) and Liberia's Forest Development Agency in a monitoring expedition funded by the People's Trust for Endangered Species captured the elusive pygmy hippo on film within three days.
The cameras will remain in place as part of a monitoring programme to produce an accurate population estimate and a conservation plan, the researchers said.
Ben Collen, ZSL research fellow said the team had headed to Liberia, which had suffered two civil wars in the last two decades, to search for the "extraordinary, mysterious creature".
"We were delighted to discover that a population still persists there, but remain highly concerned for the species, which continues to face significant threats from poaching and habitat degradation," he said.
FFI senior projects manager Africa Stephen van der Mark said: "Liberia's Upper Guinean forest ecosystem, where the pygmy hippo was photographed, is a major global conservation priority.
"Though unsustainable forestry and mining operations were especially devastating during the civil wars, they still pose a significant threat today.
"Only 10 per cent of the original Upper Guinean forest is left, of which Liberia contains about 40 per cent.
"This new sighting gives us hope that, with a concerted effort, we can still protect the area's remaining biodiversity including the charismatic pygmy hippo," he said.
Liberia's Forestry Development Authority's managing director John T Woods said he had been "pessimistic" about the existence of the pygmy hippo at Sapo, but the photos confirmed the richness of biodiversity in the Liberian forest.