This randomized controlled trial evaluated the effect of a mindfulness-based mobile health (mHealth) intervention, tailored to the pandemic context, among young adult students (N = 114) with elevated anxiety and/or depressive symptoms during quarantine in China, compared to a time- and attention-matched social support-based mHealth control. At baseline, postintervention (1 month), and 2-month follow-up, participants completed self-reports of primary outcomes (anxiety and depression), secondary outcomes (mindfulness and social support), and emotional suppression as a culturally relevant mechanism of change. Feasibility and acceptability were also evaluated. Using intent-to-treat (ITT) analysis, linear mixed effects models showed that compared to social support mHealth, mindfulness mHealth had a superior effect on anxiety (p = .024, between-group d = 0.72). Both conditions improved on depression (baseline-to-FU ds > 1.10, between-group difference not significant, d = 0.36 favoring mindfulness). There was an interaction of Emotional suppression reduction × Condition in the improvement of anxiety and depression. Further, mindfulness mHealth was demonstrated to be more feasible and acceptable in program engagement, evaluation, skills improvement, and perceived benefit. Retention was high in both conditions (>80%). The difference in self-reported adverse effect was nonsignificant (3.9% in mindfulness and 8.7% in social support). Results of this pilot trial suggest that both mindfulness and social support, delivered via mHealth, show promise in reducing distress among young adults in quarantine, with mindfulness being particularly effective in addressing anxiety. Successful implementation and dissemination of this mHealth intervention approach have the potential for addressing the psychological consequences of the pandemic. (PsycInfo Database Record (c) 2022 APA, all rights reserved).