Aim: We are presenting a case report of an adult-onset Niemann– Pick disease type C (NP-C) presenting as a bipolar-affective disorder. Background: The NP-C is a rare autosomal recessive lysosomal storage disorder. Clinical presentations of NP-C feature a range of systemic and neurological signs that arise at different ages and progress at different rates. Many of the neurological and psychiatric signs of the disease are shared with other neurological conditions, and NP-C is therefore often overlooked. Case Description: A 43-year male was under psychiatric treatment for the last 20 years without any improvement in the diagnosis of “bipolar-affective disorder.” Presence of vertical gaze palsy with cerebellar ataxia, extrapyramidal symptoms and cognitive decline over a long progressive course of illness made Niemann–Pick (NP) disease as a possible diagnosis. The diagnosis was confirmed by low sphingomyelinase levels. Conclusion: Early diagnosis of the disease is helpful in providing appropriate psychiatric and neurological management along with genetic counseling. To ensure rapid diagnosis and proper management, it is important that clinicians are familiar with this obscure condition. Clinical significance: The gradual progression of NP-C poses a diagnostic challenge. This patient was managed for psychiatric illness without improvement. The gradual appearance and progression of neurological symptoms was either masked by the behavioral disturbances or was attributed to the use of psychotropic medication. The psychiatric manifestations can overshadow the subtle neurological signs of NP-C, which leads to significant delays in the detection and diagnosis of NP-C.