A module has recently been developed within a mobile application (app) with the purpose of addressing dysfunctional beliefs associated with eating disorders (ED). The aim was to conduct a preliminary efficacy study by analyzing changes after use of the app in a general adult population. A quasi-experimental design was used in which 86 people completed a baseline assessment, and of them, 32 used the app daily for 3 minutes over a period of 15 days and completed the post-treatment assessment. Results before and after use of the app showed a decrease in mean scores for dysfunctional beliefs about perfectionism associated with physical appearance, vulnerability to weight gain, and the importance of thought control, as well as an increase in self-esteem. No significant reduction in eating symptomatology or depressive symptoms was observed. The results suggest that the app could be useful in reducing dysfunctional beliefs associated with ED.