Background: Despite the efficacy of oral adjuvant endocrine therapy (AET) for improving survival among women with hormone receptor-positive early-stage breast cancer, adherence rates remain low. Our THRIVE study (NCT03592771) investigated the effectiveness of a mobile health remote monitoring app with and without tailored educational messages on AET adherence among women with breast cancer and found no statistically significant treatment effects on 1-year AET adherence overall. Health literacy contributes to patients’ understanding of their treatment plans. We conducted a post hoc analysis of the THRIVE study to examine if health literacy moderates the effectiveness of this intervention. Methods: This non-blinded randomized controlled trial included women with early-stage breast cancer prescribed AET at a large cancer center with 14 clinics across three states. Participants used a pillbox to electronically monitor AET adherence for 1 year and completed surveys at enrollment. Consented participants were randomized into (1) “App”, receiving access to the study adherence and symptom monitoring app for 6 months, with increasing/severe symptoms and missed doses reported in the app triggering follow-ups from the oncology team; (2) “App+Feedback”, receiving additional weekly text messages about managing symptoms, adherence, and communication for 6 months; or (3) “Enhanced Usual Care (EUC).” The primary outcome was 1-year AET adherence captured with the pillbox (≥80% Proportion of Days Covered [PDC] vs. < 80%). The enrollment survey captured participant’s sociodemographic characteristics, including race and ethnicity, education, household income, and health literacy. We used a linear probability model to measure the interaction between the study arm and health literacy on AET adherence. Results: Among 304 women randomized (104 EUC, 98 App, and 102 App+Feedback), the 12-month follow-up retention rate was 88% (n = 266) and 19.4% reported low health literacy at enrollment. Low health literacy was more prevalent among Black (29.4%) vs. White (13.4%) participants (p < 0.001), those with incomes below the federal poverty level (34.4% vs. 17.2% of those with higher incomes, p = 0.02), and those with only a high school degree or lower education (31.7%) vs. with those with some college or higher education (16.4%, p < 0.01). In the low health literacy group, 80.0% of App+Feedback were AET adherent vs. 42.1% of EUC, a 37.9 percentage point (ppt) difference (95% CI: 4.1 to 71.7, p= 0.03); in higher literacy group adherence, 47.1% of App+Feedback vs. 59.0% of EUC were adherent, a -11.8 ppt difference (95% CI: -27.9 to 4.3, p= 0.15). Conclusions: A remote monitoring app with tailored educational text messages led to higher 1-year AET adherence among participants with low health literacy, but not for those with high health literacy. Clinical trial information: NCT03592771.