Social comparison has a significant impact on individuals' motivation, affect, and behavior. However, we lack a scale that captures individual differences in attitudes toward social comparison. To address this gap, we developed the Attitudes Toward Social Comparison Inventory (ASCI) drawing on existing scales that tap into metacognitive beliefs about worrying, self-motives, beliefs about emotions, and the general comparative-processing model. We examined the psychometric properties of the ASCI in a longitudinal study (N = 1,084), and a second (N = 550) and third cross-sectional study (N = 306). Through exploratory and confirmatory factor analyses, we identified a 12-item two-factor solution capturing positive and negative attitudes toward social comparison. The ASCI demonstrated measurement invariance across gender and time. The two factors were differentially and longitudinally associated with relevant constructs, including social comparison, metacognitive beliefs about worrying, depression, self-concept clarity, envy, and self-esteem. The ASCI facilitates comprehensive investigations of social comparison processes.