Depression is one of the major challenges worldwide, with significant increasing personal, economic, and societal costs. Although empirically supported treatments have been developed, they are not always available for patients in routine clinical care. Even when patients are adequately treated, they do not always respond to treatment and/or face a significant risk for recurrence. Therefore, effective and wide accessible strategies to prevent the onset of the very first depressive symptoms are necessary. Mental health apps could prove a valuable alternative for that. Although preliminary research has indicated that apps could be useful in treating depression, no study has attempted to test their utility in preventing depression symptoms. The current study aims to contrast the efficacy of a smartphone app in preventing depression compared against a waiting list condition. The app includes self-help materials and exercises based on cognitive behavioral therapy (CBT) for depression, presented in a tailored manner and incorporating gamification elements aimed at boosting the motivation to use the app.