Background Unipolar mania (UM), in which only manic episodes occur during the course of illness, may be an important clinical manifestation of bipolar disorder that is under‐recognized and understudied. The aim of this study is, for the first time, to examine the prevalence of UM and its clinical characteristics in the community. Methods Among a total of 1,411 subjects with bipolar I disorder, we evaluated the prevalence of UM using three different criteria proposed in previous studies. We compared the clinical characteristics of subjects with UM to those with a more classic bipolar presentation with mania and lifetime major depressive episode (MDE). We additionally explored the proportion of subjects with UM who later experience at least one MDE during a 3‐year follow‐up period and determined risk factors for converting from UM to classic bipolar disorder. Results The prevalence rate of UM among those with bipolar disorder ranged from 5.0 to 7.2% depending on the criteria. UM was more common in male and nonwhite subjects. About half of individuals with UM experienced subthreshold depression. Individuals with UM had lower rates of comorbid anxiety disorders or attention deficit hyperactivity disorder (ADHD). During the follow‐up, about 18% of subjects with UM experienced MDEs. Male, nonwhite, comorbid generalized anxiety disorder and ADHD predicted an increased transition from UM to classic bipolar disorder. Subthreshold depression was not associated with the risk of the transition. Conclusions UM is an infrequent but clinically distinct subtype of bipolar I disorder. Further research delineating the characteristics of UM is warranted.