Abstract Background: Adjuvant hormonal therapy (HT) is highly effective and appropriate for nearly all women with hormone receptor-positive tumors, making such treatment the most widely prescribed therapy for patients with this type of breast cancer. Despite its proven benefits in reducing cancer recurrence and improving survival, HT adherence is suboptimal (less than 80%). About 33% of patients do not take their medication as prescribed and are at increased risk of disease recurrence and lower survival. Smartphone ownership has increased substantially over the past decade, providing an extraordinary opportunity for innovation in the delivery of tailored interventions to improve patients’ adherence to hormonal therapy. Purpose: We present preliminary results of a pilot study to test the feasibility of an intervention consisting of a theory-based, bilingual, culturally tailored, and interactive mobile app + patient navigation to empower patients’ self-monitoring and management by facilitating patient education, self-efficacy, early identification and reporting of side effects, delivery of self-care advice, and timely feedback through direct communication between the patient and the oncology team. Methods: This is a 2-group parallel, randomized control trial recruiting patients (120) receiving hormone therapy treatment and attending the breast clinic at the Mays Cancer Center (MCC). The intervention group receives two components: 1) the HT Helper phone app; and 2) assistance from a patient navigator who will provide educational, psychosocial support and reinforcement, address common barriers, and facilitate the interaction with the medical team as needed. The control group receives the usual care and information provided by the MCC’s breast clinic to patients undergoing HT. The app and navigation support are based on Social Cognitive Theory and principles of motivational interviewing. Patients are assessed at baseline, three and six months. The primary outcome is HT adherence. Additional variables of interest include self-efficacy, social support, depression, side effects, anxiety, and quality of life. We also assess app usability and satisfaction. Results: We have recruited 108 patients, 56 in the intervention group and 52 in the control group. The mean age is 57.5 years, 58.3% are Latinas, 41.7% have less than high school education, 54.2% have a family income of less than $50,000/year and 52.8% have Medicare/Medicaid. In addition to descriptive data, we will present results of the 3-month and 6-month follow-ups. Conclusion: The anticipated outcome of this innovative, multi-communication study is a scalable, evidence-based, and easily adaptable intervention with potentially broad use to patients using oral anticancer agents. The intervention has the potential to improve breast cancer outcomes by reducing recurrence, improving quality of life and survival, and reducing healthcare costs. The ultimate goal of this innovative, multi-communication intervention is to improve overall survival and life expectancy, enhance quality of life, and decrease healthcare costs. Citation Format: Vivian Cortez, Patricia Chalela, Armida Flores, Sandra Sivak, Edgar Munoz, Ysabel Lew, Virginia Kaklamani, Kate I. Lathrop, Devasena Inupakutika, David Akopian, Amelie Ramirez. HT Helper: A Mobile App to Promote Hormone Therapy Adherence among Breast Cancer Patients [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P6-05-25.