Leprosy is suspected to develop after a long period of latency following infection with Mycobacterium leprae (M. leprae) during infancy, but definitive proof has been lacking. We found a rare case of leprosy in a chimpanzee (Pan troglodytes) born in West Africa (Sierra Leone) and brought to Japan around 2 years of age. At 31, the ape started exhibiting pathognomic signs of leprosy. Pathological diagnosis, skin smear, serum anti-phenolic glycolipid-I (PGL-I) antibody, and by PCR analysis demonstrated lepromatous leprosy. Single-nucleotide polymorphism (SNP) analysis verified the West African origin of the bacilli. This occurrence suggests the possibility of leprosy being endemic among wild chimpanzees in West Africa, potentially posing a zoonotic risk.