Background: COVID-19 lockdowns caused widespread closures of supportive care services for breast cancer survivors in Australia. In a randomized controlled trial, our team's lifestyle-focused, evidence-based SMS text message support program (EMPOWER-SMS COVID-19) was found to be acceptable and useful for breast cancer survivors, and it was ready for rapid widespread delivery., Objective: This study aims to evaluate the reach (uptake) of an adapted 3-month lifestyle-focused SMS text message program (EMPOWER-SMS COVID-19) and barriers and enablers to implementation using the Reach, Effectiveness, Adoption, Implementation, and Maintenance (RE-AIM) framework., Methods: A mixed methods pre-post study was conducted to evaluate the EMPOWER-SMS COVID-19 program. The study evaluated the following aspects: (1) reach/representativeness, which refers to the proportion of participant enrollment (ie, number enrolled/number that visited the study website) and demographics (eg, age, sex, ethnicity, time since completing treatment, Index of Relative Socio-economic Advantage and Disadvantage [IRSAD; quintile 1, which refers to most disadvantaged areas, to quintile 5, which refers to least disadvantaged areas, and remoteness); (2) effectiveness, in which participant engagement and acceptability were evaluated using SMS text message reply data and a feedback survey (5-point Likert scale and free-text responses); (3) adoption, which corresponds to the proportion of organizations or health professionals who agreed to promote the program; (4) implementation fidelity and maintenance, which evaluated SMS text message delivery data, opt-outs, costs, and adaptations. Quantitative data were summarized using means and SDs or frequencies and percentages, while qualitative data were analyzed thematically., Results: With regard to the reach/representativeness of the program, 841/1340 (62.8%) participants enrolled and provided electronic consent. Participants had a mean age of 58.8 (SD 9.8; range 30-87) years. According to the data collected, most participants identified as female (837/840, 99.6%) and White (736/840, 87.6%) and nearly half (418/841, 49.7%) finished treatment ≤18 months ago. Most resided in major cities (574/838, 68.5%) and 30% (251/838) in IRSAD quintile 1 or 2. In terms of effectiveness, 852 replies were received from 511 unique participants (median 1; range 1-26). The most common replies were participants stating how they heard about the program (467/852, 54.8%) or "thank you" (131/852, 15.4%). None of the replies contained urgent safety concerns. Among participants who provided feedback (449/841, 53.4%), most "(strongly) agreed" the SMS text messages were easy to understand (445/448, 99.3%), useful (373/440, 84.8%), helped participants feel supported (388/448, 86.6%), and motivated participants to be physically active (312/445, 70.1%) and eat healthier (313/457, 68.5%). Free-text responses revealed 5 factors influencing engagement: (1) feeling supported and less alone, (2) motivation and reassurance for health self-management, (3) the variety of information, (4) weblinks to information and resources, and (5) the option to save the SMS text messages. Concerning adoption, 50% (18/36) of organizations/health professionals agreed to promote the program. With regard to implementation/maintenance, SMS text messages were delivered as planned (97.43% [41,257/42,344] of SMS text messages were successfully delivered) with minimal opt-outs (62/838, 7.4%) and low cost (Aus $15.40/participant; Aus $1=US $0.67). No adaptations were made during the intervention period. Postintervention adaptations included adding weblinks and participant-selected customizations., Conclusions: EMPOWER-SMS COVID-19 was implemented quickly, had a broad reach, and had high engagement and acceptability among socioeconomically diverse participants. The program had high fidelity, low cost, and required minimal staff oversight, which may facilitate future implementation. However, further research is needed to evaluate barriers and enablers to adoption and implementation for health professionals and strategies for long-term maintenance., (©Anna C Singleton, Rebecca Raeside, Karice K Hyun, Molly Hayes, Kerry A Sherman, Elisabeth Elder, Julie Redfern, Stephanie R Partridge. Originally published in the Journal of Medical Internet Research (https://www.jmir.org), 25.07.2023.)