Bipolar disorder is characterized by disruptions in mood and affect that occur not only during mood episodes, but during euthymic periods as well. At the same time, sensitivity of the behavioral activation system (BAS) has been implicated in the disorder and is a risk marker for it. Less clear is the relationship between BAS sensitivity and positive affect, particularly lower level facets of positive affect. The aim of the present study was to examine the relationship between positive affect and vulnerability for mania as assessed using BAS sensitivity. Specifically, the link between daily levels and fluctuations of positive affect and baseline BAS sensitivity was examined. Following the hierarchical model of affect, this study also assessed the relationship between BAS sensitivity and the distinct facets of positive affect. Finally, this study examined whether BAS sensitivity moderates associations between daily rewards and positive affect. Undergraduates (N = 265) from a large university in the South were recruited to complete measures of BAS sensitivity, affect, and mood symptoms at baseline. Using ecological momentary assessment (EMA), participants completed daily surveys assessing affect and engagement with rewarding situations. An exploratory factory analysis revealed a four factor structure of positive affect, consisting of Serenity, Joviality, Attentiveness, and Self-Assurance. Greater daily levels of overall positive affect, as well as the lower order facets of Joviality, Self-Assurance, and Attentiveness, were predicted by heightened BAS sensitivity. In contrast, the facet of Serenity demonstrated minimal associations with BAS sensitivity. The study findings support a multi-faceted structure of positive affect and suggest that certain facets may be more closely related to risk for bipolar disorder. Specifically, Joviality and Self-Assurance may represent maladaptive forms of positive affect, whereas Serenity may function as a protective element against bipolar disorder.