Mirror exposure (ME) is one of the main components of the treatment of patients with eating disorders symptomatology and it has shown its effectiveness in improving several outcomes (e.g., body dissatisfaction). However, the study as to what body posture should be adopted to maximize its effectiveness has been neglected. From embodied cognition and emotion theories, the adoption of an expansive (vs. contractive) body posture has been associated with positive changes in cognitive and emotional responses. The objective of this study was to analyze the effect of adopting an expansive (vs. contractive) posture before an ME task on body-related emotions and cognitions, as well as to analyze the possible moderator and mediator variables of these relationships. The sample was composed of 68 women (age: M = 21.74, SD = 3.12) with high scores on body dissatisfaction. Participants were randomly assigned to the expansive or contractive condition, where the openness of the arms/legs and the back position were manipulated. Posture was monitored by an electronic device and participants filled out several self-reported measures. ANCOVAs, moderation, mediation, and moderated mediated analyses were performed. Results showed that women in the expansive condition showed higher positive emotions after the ME. Moreover, exploratory analyses showed that adopting an expansive posture improved positive emotions, leading to improvements in negative emotions, body image satisfaction, and appraisal of the person's own body. Psychological interventions should explore the value of holding an expansive posture before the ME in women with body dissatisfaction.