High school students' lack of coping skills can be detrimental to their wellbeing, which has created a need for interventions in the educational system. A review of the literature provided evidence supporting the feasibility and effectiveness of mindfulness-based interventions for adolescents. This study examined if and to what extent a difference existed in California high school students' subjective wellbeing before and after using an online mindfulness-based application. A quasi-experimental, single-group, pretest-posttest was selected. The theoretical foundation for this study was positive psychology and the PERMA model. The sample consisted of 83 high school students (n = 83) from two convenient locations (a high school and fitness center) in California. Students completed the intervention via ZOOM for five days. Participants completed the EPOCH Measure of Adolescent Wellbeing on day one and again on day five. Data were analyzed using repeated measures MANOVA on the before-and-after difference scores (after-before) for the five subjective wellbeing subscales (engagement, perseverance, optimism, connectedness, and happiness). The overall findings of the repeated measures MANOVA were statistically significant, Roy's Largest Root = 1.07, F (5, 78) = 16.64, p < 0.001, partial [eta-squared] = 0.52, indicating that there were significant differences in wellbeing dimensions collectively before and after listening to the guided meditations application. The results provided initial evidence on the efficacy of online mindfulness-based applications to enhance high school students' subjective wellbeing. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]