Background <p>Survivorship Care Plans (SCPs) support the transition from active cancer treatment to long-term survivorship. The POSTHOC mobile application (app) was developed to digitize the SCP, improving accessibility, modifiability, longevity, and usefulness, while promoting adherence to diet and physical activity recommendations. This study tested the feasibility, acceptability, and usability of the POSTHOC app and its effect on global symptom burden compared to a traditional SCP among recent cancer survivors.</p> Methods <p>In a phase I/II randomized controlled trial, cancer survivors within 12&#xa0;weeks of curative treatment were recruited. Participants were randomized 2:1 POSTHOC:usual care. The POSTHOC arm had full use of the app for 12&#xa0;weeks, while the usual care arm received a static SCP. Outcomes were assessed at baseline, 6&#xa0;weeks, and 12&#xa0;weeks, which included symptom burden (MD Anderson Symptom Inventory), diet (ASA24, 24-h recall), and physical activity (Fitbit). Acceptability (custom usefulness scale) and usability (System Usability Scale, range 0–100) were assessed within the POSTHOC arm at weeks 6 and 12. Mixed models tested SCP effects on outcomes. At week 12, participants also reflected on patient-provider communication regarding healthy lifestyle behaviors.</p> Results <p>Fifty-one participants consented, 41 participants (80%) successfully downloaded the app, and 34 were randomized. In the POSTHOC group, 48% (95% CI: 28–68%) and 52% (95% CI: 32–72%) recorded symptoms and/or diet in the app at weeks 6 and 12, respectively, which was below the a priori 75% usage hypothesis. POSTHOC participants reported moderate usefulness of the app at week 6 (mean ± SD = 3.5 ± 2.2) and week 12 (4.3 ± 2.4). In regard to usability, POSTHOC participants rated the app 62.7 ± 21.1 at week 6 and 65.7 ± 18.1 at week 12, which is slightly below “average” in industry standards. In regard to exploratory symptom outcomes, the POSTHOC group showed a greater reduction in symptom burden than controls at week 12 [b = 3.40 (95% CI: 0.51‒6.28), <i>p</i> = 0.022, ES = 0.36]. Improvements were also observed at week 12 for memory problems [b = 0.42 (95% CI: 0.03‒0.82), <i>p</i> = 0.034; ES = 0.30] and symptom interference with walking [b = 0.47 (95% CI: 0.05‒0.90), <i>p</i> = 0.030; ES = 0.45]. On average, participants desired more communication about healthy behaviors, reporting dissatisfaction with these conversations on average [2.6 (95% CI 1.5‒3.8), range 1–5].</p> Conclusions <p>POSTHOC is a partially feasible digital SCP with moderate usability and potential to reduce symptom burden in cancer survivors, supporting mobile health approaches to survivorship care. Future studies should explore the effects of a mobile SCP on symptom outcomes as a primary outcome.</p> Trial Registration <p>ClinicalTrials.gov: NCT05499663.</p> Graphical Abstract <p></p>