Purpose: This study aimed to develop a health belief model-based smartphone breast self-examination (BSE) education application and to evaluate its usability through expert assessment. Methods: A methodological design was used, and the application was developed in three stages: content development, application design, and implementation. The application included educational materials, step-by-step BSE instructions, video-based guidance, Q&A, and chatbot-based communication. Usability was evaluated by seven experts, including two nurses working at a breast cancer center, three nursing professors with application development experience, and two information technology experts. Data were analyzed using descriptive statistics. Results: The final application had six main components: main screen, survey, educational materials, educational video, Q&A, and chatbot-based communication function. The total usability score was 77.00±2.00 out of 80, and the mean item score was 4.81±0.13 out of 5. The mean scores for content accuracy, accessibility and convenience, speed and connection, and overall impression were 19.29±0.76, 28.71±0.49, 14.71±0.49, and 14.29±0.76, respectively. Conclusion: The health belief model-based smartphone BSE education application showed high expert-rated usability. The results provide preliminary evidence for applying smartphone-based BSE education in community settings.