Background: Breast cancer diagnosis and its treatment act as a life-threatening negative stressor seriously affecting the physical and psychological wellbeing of the female geriatric patients. Although there are many evidences that support the value of patient education in the management of breast cancer and consecutive treatment side effects, few studies determine the effect of applying mobile based education on self-efficacy and psychological distress among female geriatric patients with breast cancer who undergo chemotherapy. Aim of the study: To determine the effect of mobile based educational program through WhatsApp on self-efficacy and psychological distress among female geriatric patients with breast cancer undergoing chemotherapy. Setting: The study was conducted in the chemotherapy outpatient clinics of Damanhour Oncology Institute in El-Behaira Governorate, Egypt. Subjects: A convenience sample of sixty female geriatric patients with breast cancer was randomly divided into two equal groups. Design: The study followed a quasi-experimental research design (study and control groups).Tools: Four tools were used for data collection:1) Socio-demographic and Clinical Data Structured Interview Schedule,2) Cancer Treatment Survey (CaTS), 3) The Side effects-Management Self-Efficacy Scale-Breast Cancer (SMSES-BC) Related to Chemotherapy Questionnaire, and 4) National Comprehensive Cancer Network Distress Thermometer (NCCN-DT) Results: After the interventions, the mean scores of side effect management self-efficacy were increased from (27.07 ± 34.89)  to (172.47 ± 64.05) with a statistically significant difference  between pre and post interventions in the study group (P= 0.000). As well, psychological distress mean scores were reduced from (6.600 ± 2.343) to (1.370 ± 1.752) with a statistically significant difference between pre and post interventions in the study group (P= 0.000).A statistically significant differences were found between the study and control groups post interventions regarding side effect management self-efficacy and psychological distress (P=0.000).Conclusion: Implementing the mobile based educational program through WhatsApp significantly   improves self-efficacy and reduces psychological distress among female geriatric patients with breast cancer undergoing chemotherapy. Recommendations: The oncology nurses should implement mobile based educational sessions for female geriatric patients with breast cancer undergoing chemotherapy to improve their self-efficacy in managing chemotherapy side effects and enhance their psychological status.