Similar to effects identified with traditional media forms, recent evidence indicates that body image concerns, such as body dissatisfaction and drive for thinness, may also be associated with exposure to images on Social Networking Sites. Utilizing social comparison theory, the current study sought to examine the relationship between female university students' photo-based activities on Instagram, which is a relatively new Social Networking Site, appearance-related comparisons, and two outcome variables: drive for thinness and body dissatisfaction. Mediational analyses using bootstrapping methods indicated that Instagram photo-based activities positively predicted both drive for thinness and body dissatisfaction through the mediating variable of appearance-related comparisons. These results suggest that Instagram use could be potentially harmful to individuals who find themselves frequently engaging in comparisons with others. Additionally, utilizing the intrasexual competition framework, the second aim of this study was to determine whether individual differences in competitiveness for mates influenced individual tendencies to engage in appearance-related comparisons on Instagram. A significant positive relationship emerged between intrasexual competitiveness for mates and appearance-related comparisons on Instagram. Theoretical and applied implications from these findings are discussed. •Surveyed undergraduate students on Instagram photo activities as the predictor variable.•Appearance-related comparisons was the mediator variable and intrasexual competition was the moderator variable.•Drive for thinness and body dissatisfaction were criteria.•Significant mediating effect for both criteria was found.•Adds to the body of literature pertaining to body image concerns and SNS use.