Background and aims: Problematic usage of the internet (PUI) has been linked to impaired mental health and academic functioning in young adults. This randomized controlled trial evaluated the efficacy of a 3-week mobile-based approach–avoidance task (AAT) training (PROTECTapp) for reducing PUI in university students. Methods: Ninety-two participants (Mage = 22.00 years, 69.6% women) with elevated levels of PUI were randomized to the PROTECTapp intervention (n = 45) or a waitlist control group (n = 47). Primary outcomes were PUI severity and internet-related craving. Secondary outcomes included motivation to change, psychopathological symptoms and academic functioning. Participants were assessed at baseline and postintervention; the intervention group completed additional 3- and 12-week follow-ups. Results: Intention-to-treat analyses indicated greater reductions in PUI following the PROTECTapp intervention compared to the waitlist (p = .003; d = -0.80, 95% CI [-1.23, -0.38]). No significant effects emerged for craving or broader psychological outcomes (ps > .05), though favorable effects were observed on motivation to change (ambivalence: p = .020; d = -0.24, 95% CI [-0.65, 0.17]; taking steps: p = .002; d = 0.45, [0.04, 0.87]). Satisfaction with the intervention was moderate (M = 18.32 of 32), and participants completed on average 35.52 training sessions. Adverse events were reported infrequent (7.1%). Discussion and Conclusions: PROTECTapp is a promising mobile-based intervention to reduce PUI and enhance motivation to change in young adults. Its brevity, scalability, and safety profile highlight its potential as a low-threshold preventive or adjunctive intervention for young individuals at-risk.