Although psychological distress is common among survivors of critical illness, there are few tailored therapies. To determine the optimal method for delivering a mindfulness intervention via a mobile app for critical illness survivors. This randomized clinical trial used a 2 × 2 × 2 factorial design and was conducted at 3 sites among survivors of critical illness with elevated postdischarge symptoms of depression. The study was conducted between August 2019 and July 2023. Participants were randomized to 1 of 8 different groups as determined by 3 two-level intervention component combinations: intervention introduction method (mobile app vs therapist call), mindfulness meditation dose (once daily vs twice daily), and management of increasing symptoms (mobile app vs therapist call). The primary outcome was the 9-item Patient Health Questionnaire (PHQ-9) depression scale score (range, 0-27) at 1 month. Secondary outcomes included anxiety (7-item Generalized Anxiety Disorder) and posttraumatic stress disorder (Posttraumatic Stress Scale) symptoms at 1 and 3 months, adherence, and feasibility. General linear models were used to compare main effects and interactions of the components among intervention groups. A formal decisional framework was used to determine an optimized intervention version. A total of 247 participants (mean [SD] age, 50.2 [15.4] years; 104 [42.1%] women) were randomized. Twice-daily meditation compared with once-daily meditation was associated with a 1.2 (95% CI, 0.04-2.4)-unit lower mean estimated PHQ-9 score at 1 month and a 1.5 (95% CI, 0.1-2.8)-unit lower estimated mean score at 3 months. The other 2 intervention components had no main effects on the PHQ-9. Across-group adherence was high (217 participants [87.9%] using the intervention at trial conclusion) and retention was strong (191 [77.3%] and 182 [73.7%] at 1 and 3 months, respectively). A mindfulness intervention for survivors of critical illness that included an app-based introduction, twice-daily guided meditation, and app-based management of increasing depression symptoms was optimal considering effects on psychological distress symptoms, adherence, and feasibility. ClinicalTrials.gov Identifier: NCT04038567.