Impulsivity is a core feature of many psychiatric disorders. Traditionally, impulsivity has been assessed using retrospective questionnaires or laboratory tasks. Both approaches neglect intraindividual variability in impulsivity and do not capture impulsivity as it occurs in real-world settings. The goal of the current study was to provide a method for assessing impulsivity in daily life that provides both between-individual and within-individual information. Participants with borderline personality disorder (BPD; n = 67) or a depressive disorder (DD; n = 38) carried an electronic diary for 28 days and responded to 9 impulsivity items up to 6 times per day. Item distributions and iterative exploratory factor analysis (EFA) results were examined to select the items that best captured momentary impulsivity. A brief 4-item scale was created that can be used for the assessment of momentary impulsivity. Model fit was good for both within- and between-individual EFA. As expected, the BPD group showed significantly higher scores on our Momentary Impulsivity Scale than the DD group, and the resulting scale was moderately correlated with common trait impulsivity scales.