This experimental study examined the effects of engaging on social media with attractive female peers on young adult women’s body image. Participants were 118 female undergraduate students randomly assigned to one of two experimental conditions. Participants first completed a visual analogue scale measure of state body image and then either browsed and left a comment on the social media site of an attractive female peer (n = 56) or did the same with a family member (n = 62) and then completed a post-manipulation visual analogue scale measure of state body image. A 2 × 2 mixed analysis of variance showed a significant interaction between condition and time. Follow-up t-tests revealed that young adult women who engaged with an attractive peer on social media subsequently experienced an increase in negative body image (dependence-corrected d = 0.13), whereas those who engaged with a family member did not (dependence-corrected d = 0.02). The findings suggest that upward appearance comparisons on social media may promote increased body image concerns in young adult women.