BackgroundBreast cancer, the most commonly diagnosed cancer and second leading cause of cancer-related death in women in the United States, disproportionately affects women from minoritized or low socioeconomic backgrounds. The average woman has an approximately 12% lifetime risk of developing breast cancer. Lifetime risk nearly doubles if a woman has a first-degree relative with breast cancer, and the risk increases as multiple family members are affected. Decreasing sedentary behaviors through moving more and sitting less reduces breast cancer risk and improves outcomes for cancer survivors and healthy adults. Digital health solutions, such as mobile apps that are culturally appropriate, designed with input from the target audience, and include social support, are effective at improving health behaviors.ObjectiveThis study aimed to develop and evaluate the usability and acceptability of a prototype app designed with a human-centered approach to promote moving more and sitting less in Black breast cancer survivors and their first-degree relatives (parent, child, or sibling).MethodsThis 3-phase study consisted of app development, user testing, and evaluation of user engagement and usability. Key community stakeholders were engaged in the first 2 (qualitative) phases to provide input into developing the prototype app (MoveTogether). After development and user testing, a usability pilot was conducted. Participants were adult breast cancer survivors who identified as Black and agreed to participate with a relative. Participants used the app and a step-tracking watch for 4 weeks. App components included goal setting and reporting, reminders, dyad messaging, and educational resources. Usability and acceptability were assessed with a questionnaire that included the System Usability Scale (SUS) and semistructured interviews. Data were analyzed with descriptive statistics and content analysis.ResultsParticipants in the usability pilot (n=10) were aged 30 to 50 years (6/10, 60%), not married (8/10, 80%), and college graduates (5/10, 50%). The app was used on average 20.2 (SD 8.9) out of 28 days—SUS score of 72 (range 55-95)—and 70% (7/10) agreed that the app was acceptable, helpful, and gave them new ideas. Additionally, 90% (9/10) found the dyad component helpful and would recommend the app to friends. Qualitative findings suggest that the goal-setting feature was helpful and that the dyad partner (buddy) provided accountability. Participants were neutral regarding the cultural appropriateness of the app.ConclusionsThe MoveTogether app and related components were acceptable for promoting moving more in dyads of breast cancer survivors and their first-degree relatives. The human-centered approach, which involved engaging community members in the development, is a model for future technology development work. Future work should be done to further develop the intervention based on the findings and then test its efficacy to improve sedentary behavior while considering culturally informed strategies for adoption and implementation within the community.Trial RegistrationClinicalTrials.gov NCT05011279; https://clinicaltrials.gov/ct2/show/NCT05011279