Individuals tend to compare themselves with those whose attributes and abilities are better (i.e., upward social comparison) or worse (i.e., downward social comparison) than their own to evaluate self. Based on social comparison theory, this study examined the influence of fitness comparison on fitness app users' beliefs about (self-efficacy), attitudes toward (motivation), and behaviors in (participation) physical activities (PA). For the research, fitness app users who use social media were recruited (N = 567) through a crowdsourcing internet marketplace. The results revealed that downward fitness comparison appeared to thwart fitness app users' self-efficacy in PA, whereas upward fitness comparison appeared to enhance self-efficacy in PA. As a consequence, people who engaged in upward fitness comparison had elevated motivation and participation in PA. In particular, self-efficacy was a crucial variable in improving people's motivation for PA. Those who want to improve their fitness activities should engage in comparing themselves with high performers. The given managerial implication is that fitness practitioners should encourage people to compare their fitness abilities with better performers or role models, and fitness app developers should design apps that permit users to have more opportunities for comparison with superior others.