Considering the widespread use of smartphones and their applications (apps), as well as the undeniable role of reminders and apps in behavioral interventions, this study aimed to assess the efficacy of a smartphone app (Brush DJ) for oral hygiene compliance of patients with fixed orthodontic appliances. In this randomized clinical trial, 120 patients between 15 to 25 years who had just started fixed orthodontic treatment were randomly divided into two groups (n = 60). Control patients received conventional oral hygiene instruction, while patients in the intervention group were asked to use the Brush DJ smartphone app, after receiving conventional oral hygiene instruction. The plaque index (PI) and gingival index (GI) were measured at baseline (T0), and at 4 weeks (T1), 8 weeks (T2) and 12 weeks (T3) after the onset of study. A questionnaire was given to all patients to assess the frequency and duration of tooth brushing per day, and the frequency of app usage and reminder noticing in the intervention group. Improvements in PI and GI were noted in the intervention group; while these parameters increased in the control group. Significant differences were noted in PI and GI changes between the two groups (p < 0.001). Brushing frequency and duration were positively correlated with app usage during the follow-up period. Ultimately, we believe that smartphone apps, as motivators and reminders, can greatly help in improving the orthodontic patients’ oral hygiene compliance, especially in adolescents