This study investigated how social media use is related to body esteem (particularly weight satisfaction and muscularity importance) in adolescents. We conducted a 17-day Ecological Momentary Assessment study among 183 adolescents (12–17 years, 58% girls). Each adolescent reported on his/her social media use, weight satisfaction, and muscularity importance four times per day (68 assessments per participant; 6,863 completed in total). Using a person-specific, N=1 method of analysis (Dynamic Structural Equation Modeling), we found that at the between-person level, social media use is not associated with lower satisfaction with one’s weight and greater importance of looking masculine. At the within-person level, we found a significantly negative association for weight satisfaction and a significantly positive association for muscularity importance. For weight satisfaction, 76% of adolescents experienced no or very small effects as a result of SMU, 2% experienced positive effects, and 22% experienced negative effects. Regarding muscularity importance, 89% experienced no or very small effects of SMU on muscularity importance, 10% experienced positive effects, and 1% experienced negative effects. There is little evidence of gender differences in the effect of social media on body esteem in our sample.