Body dissatisfaction is prevalent in young women, and is associated with symptoms of body dysmorphic disorder (BDD) and eating disorders (EDs). The aim of the study was to assess the efficacy of a mobile application, based on cognitive behavioral principles, in reducing body dissatisfaction and BDD/ED symptoms in female university students considered at high-risk of developing body image disorders (BIDs). Fifty university students at high-risk of developing BIDs (using self-report questionnaires assessing BIDs and the Structured Clinical Interview for DSM-5 Clinical Version) were randomized in two groups: immediate-use App group (iApp group; n = 25) and delayed-use App group (dApp group; n = 25). The iApp group started using the app at baseline for 16 days (T0 to T1). The dApp group waited for 16 days before starting to use the app (T1 to T2). Participants completed questionnaires at baseline (T0), 16 days from baseline (T1), and 32 days from baseline (T2). Repeated measure Analyses of Variance (ANOVAs) showed a Group (iApp vs. dApp) × Time (T0 vs. T1) interaction on BDD symptoms indicating medium effect size (partial eta squared) reductions in the iApp group compared to dApp group at T1; post-intervention means for BDD symptoms were under the cut-off for extreme body dissatisfaction/BDD symptoms in both groups. Pertaining to ED symptoms, no significant Group × Time interaction was detected. Training 3 minutes a day for 16 days with a CBT-based mobile application may lead to reductions in some forms of body dissatisfaction, including BDD symptoms in female university students at high-risk of developing BIDs. On the other hand, effects of the intervention on ED symptoms seem more limited.