The primary aim of this study is to test the efficacy of a Smartphone-based cognitive behavioral therapy (CBT) treatment for adults with body dysmorphic disorder (BDD) recruited nationally. In a prior study (Clinical Trials Identifier # NCT03221738), the investigators developed and pilot-tested the feasibility, acceptability, and preliminary efficacy of a CBT for BDD app in an open pilot trial. The investigators are now further testing these outcomes in a randomized controlled trial. Eligible subjects (N= 64) will be randomly assigned to 12-weeks of Smartphone-delivered CBT for BDD either immediately, or after a 12-week long waiting period (50/50 chance).