PURPOSE: Adjuvant endocrine therapy (AET) reduces breast cancer morbidity and mortality; however, adherence is suboptimal. Interventions exist, yet few have improved adherence. Patient characteristics may alter uptake of an intervention to boost adherence. We examined moderators of the effect of a virtual intervention (STRIDE; #NCT03837496) on AET adherence after breast cancer. METHODS: At a large academic medical center, patients taking AET (N = 100; M = 56.1, 91% White) were randomized to receive STRIDE versus medication monitoring. All stored their medication in digital pill bottles (MEMS Caps) which captured objective adherence. Participants self-reported adherence (Medication Adherence Report Scale) at 12 weeks post-baseline. Moderators included age, anxiety, and depressive symptoms (Hospital Anxiety and Depression Scale), AET-related symptom distress (Breast Cancer Prevention Trial Symptom Scale), and AET-specific concerns (Beliefs about Medications Questionnaire). We used hierarchical linear modeling (time × condition × moderator) and multiple regression (condition × moderator) to test the interaction effects on adherence. RESULTS: Age (B = 0.05, SE = 0.02, p = 0.003) and AET-related symptom distress (B = -0.04, SE = 0.02, p = 0.02) moderated condition effect on self-reported adherence while anxiety (B = -1.20, SE = 0.53, p = 0.03) and depressive symptoms (B = -1.65, SE = 0.65, p = 0.01) moderated objective adherence effects. AET-specific concerns approached significance (B = 0.91, SE = 0.57, p = 0.12). Participants who received STRIDE and were older or presented with lower anxiety and depressive symptoms or AET-related symptom distress exhibited improved adherence. Post hoc analyses revealed high correlations among most moderators. CONCLUSIONS: A subgroup of patients who received STRIDE exhibited improvements in AET adherence. The interrelatedness of moderators suggests an underlying profile of patients with lower symptom burden who benefitted most from the intervention. STUDY REGISTRATION: NCT03837496.