Abstract Quantifying the distribution of an endangered species in space and time is crucial for planning management and conservation actions. The giant otter (Pteronura brasiliensis) is an endangered South American mammal that is considered to be extinct in the Atlantic Forest biome. Historical records of P. brasiliensis were used to reconstruct its original distribution in the Atlantic Forest biome. Based on the historical records and ecological niche models, the original distribution and amount of habitat area lost for the species were estimated. The potential impacts of dams in planning reintroductions for the species were also assessed. Twenty‐six records of P. brasiliensis made between 1769 and 1999 were found. Most of the records are concentrated in the Paraná River basin. Only six records were based on voucher specimens and photographs. Areas in the southern (Paraná state, Brazil) and central (Bahia and Minas Gerais states, Brazil) Atlantic Forest were recovered as highly suitable for the species, even after considering the negative impact of dams. The ecological niche models suggested that P. brasiliensis formerly occupied an area of 909,257 km² in the Atlantic Forest. The forested rivers, along the upper Paraná basin, in northern Argentina and southern Brazil were the last refugia of the species. Pelt trade was probably a major factor in the species' disappearance, which is evidenced by the fact that Brazilian stakeholders, in Atlantic Forest sites, restricted giant otter hunting as early as 1948. The results show that historical records and ecological niche models can be effectively combined and may guide in‐situ conservation actions by identifying potential areas for reintroductions.