Autism spectrum disorder and bipolar affective disorder exhibit partial symptomatic overlap, which can complicate accurate diagnosis and delay the implementation of appropriate treatment. Increasing research also points to the potential comorbidity of both conditions, further complicating the clinical picture, diagnostics, and therapy planning. The aim of this narrative review is to examine the literature concerning the relationship between autism spectrum disorder and bipolar affective disorder, with particular emphasis on the hypothesis that autistic traits and their associated chronic functional impairments may heighten susceptibility to the development or more severe course of bipolar affective disorder. Epidemiological data, diagnostic challenges, selected neurobiological and genetic mechanisms, as well as practical implications for diagnosis, treatment, and environmental support are discussed. A review of the literature highlights the need for cautious differential diagnosis, longitudinal observation, and interdisciplinary collaboration in the care of individuals with comorbid autism spectrum disorder and bipolar affective disorder. A better understanding of the relationship between these conditions may facilitate earlier recognition of affective symptoms in individuals on the autism spectrum and support more appropriate clinical assessment and management planning.