ABSTRACT: Depression is a major challenge 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. Therefore, we need effective and widely accessible strategies to prevent the onset of the very first depressive symptoms. Mental health apps could prove a valuable solution for this desideratum. Although preliminary research has indicated that such apps can be useful in treating depression, no study has attempted to test their utility in preventing depressive symptoms. The aim of this exploratory study is to contrast the efficacy of a smartphone app in reducing cognitive vulnerability and mild depressive symptoms, as risk factors for the onset of depression, against a wait-list condition. More specifically, we aim to test an app designed to (1) decrease general cognitive vulnerability and (2) promote engagement in protective, adaptive activities, while (3) counteracting (through gamification and customization) the tendency of premature dropout from intervention.Romanian-speaking adults (18 years and older) with access to a computer and the Internet and who own a smartphone are included in the study. Two parallel randomized clinical trials are conducted: in the first one, 50 participants free of depressive symptoms (i.e., who obtain scores ?4 on the Patient Health Questionnaire, PHQ-9) will be included, while in the second one 50 participants with minimal depressive symptoms (i.e., who obtain PHQ-9 scores between 5 and 9) will be included. Participants undergoing therapy, presenting with substance abuse problems, psychotic symptoms, and organic brain disorders, or serious legal or health issues that would prevent them from using the app, as well as participants reporting suicidal ideation are excluded. Participants randomized to the active intervention will autonomously use the smartphone app for 4 weeks, while the others will be given access to the app after 4 weeks from randomization. The primary outcomes are (1) cognitive vulnerability factors as defined within the cognitive behavioral therapy (CBT) paradigm (i.e., dysfunctional cognitions, irrational beliefs, and negative automatic thoughts) (for the first trial), and (2) level of depressive symptomatology (for the second trial). The app includes self-help materials and exercises based on CBT for depression, presented in a tailored manner and incorporating gamification elements aimed at boosting motivation to use the app.This study protocol is the first to capitalize on the ubiquity of smartphones to large-scale dissemination of CBT-based strategies aimed at preventing depression in non-clinical populations.ClinicalTrials.gov: NCT02783118 . Registered on 26 May 2016.