In this theoretical review paper, we provide a developmental–sociocultural framework for the role of social media (SM) in adolescent girls’ body image concerns, and in turn, depressive symptoms and disordered eating. We propose that the features of SM (e.g., idealized images of peers, quantifiable feedback) intersect with adolescent developmental factors (e.g., salience of peer relationships) and sociocultural gender socialization processes (e.g., societal over-emphasis on girls’ and women’s physical appearance) to create the “perfect storm” for exacerbating girls’ body image concerns. We argue that, ultimately, body image concerns may be a key mechanism underlying associations between adolescent girls’ SM use and mental health. In the context of proposing this framework, we provide empirical evidence for how SM may increase adolescent girls’ body image concerns through heightening their focus on (1) other people’s physical appearance (e.g., through exposure to idealized images of peers, celebrities, and SM influencers; quantifiable indicators of approval); and (2) their own appearance (e.g., through appearance-related SM consciousness; exposure to idealized self-images; encouraging over-valuing of appearance; and peer approval of photos/videos). Our framework highlights new avenues for future research on adolescent girls’ SM use and mental health, which recognize the central role of body image.