<p><i>Leishmania infantum</i> infection has recently been identified in non-human primates in Spain, including clinically affected individuals from critically endangered species such as the Bornean orangutan (<i>Pongo pygmaeus pygmaeus</i>). Despite these emerging cases, clinicopathological data on leishmaniosis in captive wildlife remain limited. Here, we report the first confirmed case of clinical visceral leishmaniosis in a zoo-captive white-naped mangabey (<i>Cercocebus lunulatus</i>) in Spain. The affected animal presented with progressive weight loss, generalized lymphadenopathy, polyclonal gammopathy, and cytological evidence of pyogranulomatous lymphadenitis containing intralesional protozoal amastigotes consistent with <i>Leishmania</i> spp. The animal eventually died, and a detailed necropsy was performed. Moreover, to confirm <i>L. infantum</i> infection, histopathological analysis, serological assays, including immunofluorescent antibody test (IFAT), Western blot (WB), and enzyme-linked immunosorbent assay (ELISA), as well as polymerase chain reaction (PCR) and immunohistochemical analysis of affected tissues were conducted. Gross pathological examination revealed disseminated visceral involvement, characterized by multiple white nodules in the lungs, spleen, liver, and lymph nodes. Microscopically, these lesions corresponded to areas of coagulative necrosis surrounded by macrophage-rich inflammatory infiltrates, many of which contained protozoal amastigotes compatible with <i>Leishmania</i> spp. <i>L. infantum</i> infection was confirmed by all aforementioned serological tests, PCR, and immunohistochemistry. This case highlights the expanding host range of <i>L. infantum</i> and underscores the need to include leishmaniosis in the differential diagnosis of primates in zoological institutions, especially in endemic regions.</p>