Media’s prevailing thin-body ideal plays a vital role in adolescent girls’ body image development, but the co-occurring impact of peer feedback is understudied. The present study used functional Magnetic Resonance Imaging (fMRI) to test media imagery and peer feedback combinations on neural activity related to thin-body ideals. Twenty-four healthy female late adolescents rated pre-categorized body sizes of bikini models (‘too thin’ or ‘normal’), directly followed by ostensible peer feedback (‘too thin’ or ‘normal’). Consistent with prior studies on social feedback processing, results showed increased brain activity in the dorsal medial prefrontal cortex (dmPFC)/anterior cingulate cortex (ACC) and bilateral insula in incongruent situations: when participants rated media models’ body size as ‘normal’ while peer feedback indicated the models as ‘too thin’ (or vice versa). This effect was stronger for girls with lower self-esteem. A subsequent behavioral study (N=34 female late adolescents, separate sample) demonstrated that participants changed behavior in the direction of the peer feedback: pre-categorized ‘normal’ sized models were rated as ‘too thin’ more often after receiving ‘too thin’ peer feedback. This suggests that the neural responses upon peer feedback may influence subsequent choice. Our results show that media-by-peer interactions have pronounced effects on girls’ body-ideals.