Objective: The present study aimed to evaluate the efficacy of a mindfulness-based cognitive therapy (MBCT) intervention in reducing problematic mobile phone use, depression, and sleep disorders among adolescents. Additionally, it sought to investigate whether the decrease in problematic mobile phone use acted as a mediator in the relationship between the MBCT intervention and adolescent depression and sleep disorders. Methods: In a randomized controlled trial, a total of 104 adolescents were randomly assigned to the mindfulness group (n 5 52) or the wait-list control group (n 5 52). The mindfulness group students completed eight 45-min sessions of mindfulness training in four weeks. The outcomes were measured at baseline, postintervention, and at the 2-month follow-up. Results: Compared with the control group, the mindfulness group had significantly greater levels of mindfulness and lower levels of problematic mobile phone use, depression, and sleep disorders postintervention. The intervention effects were maintained at the 2-month follow-up. In addition, decreased problematic mobile phone use significantly mediated the association between the MBCT intervention and decreased depression and decreased sleep disorders. Conclusion: The findings suggest that MBCT could improve adolescent depression and sleep disorders and that decreasing problematic mobile phone use is an effective pathway accounting for the MBCT intervention effect on adolescent depression and sleep disorders.  Kulcsszavak: mindfulness-based cognitive therapy, mindfulness, problematic mobile phone use, depression, sleep disorders, adolescents