Background: Patients with cancer are increasingly prescribed oral therapies, bearing greater responsibility for self-management of treatment adherence and adverse events. We conducted a randomized trial to test the use of a smartphone mobile app to improve symptoms and adherence to oral cancer therapy. Materials and Methods: From February 18, 2015, through December 31, 2016, 181 patients with diverse cancers who were prescribed oral therapy were randomized to receive either the smartphone mobile app or standard care. The mobile app included a medication plan with reminders, a symptom-reporting module, and patient education. Primary outcomes were adherence (per electronic pill caps), symptom burden (per MD Anderson Symptom Inventory), and quality of life (per the Functional Assessment of Cancer Therapy–General). Participants also completed self-report measures of medication adherence, anxiety and depression symptoms, social support, quality of care, and healthcare utilization. Linear regression was used to assess intervention effects on adherence and change in self-report outcomes from baseline to week 12, controlling for baseline scores and social support. Results: Study groups did not differ across any outcome measure, with an overall mean adherence of 78.81% (SD, 26.66%) per electronic pill caps. However, moderation analyses showed that intervention effects on the primary adherence measure varied by baseline self-reported adherence and anxiety symptoms. Specifically, adherence rates per electronic pill caps were higher in patients randomized to the mobile app versus standard care within the subsamples of patients who reported baseline adherence problems (mean difference, –22.30%; 95% CI, –42.82 to –1.78; P=.034) and elevated anxiety (mean difference, –16.08%; 95% CI, –31.74 to –0.41; P=.044). Conclusions: Although the mobile app may not improve outcomes for all patients prescribed oral cancer therapy, the intervention may be beneficial for those with certain risk factors, such as difficulties with adherence or anxiety.