Integrating mobile technology into health promotion strategies has the potential to support healthy behaviors. A new theory-informed app was designed to augment an intervention promoting wellness motivation in older adults with fall risk and low levels of physical activity. The app content was evaluated for clarity, homogeneity, and validity of motivational messages; both the app and device were evaluated for acceptability and usability. The initial evaluation included nine adults (mean age, 75); four of whom also assessed the app's sensing abilities in the field. As part of an intervention feasibility study, 14 older adults (mean age, 84) also provided a follow-up evaluation of app usability. Evaluation participants assessed the app as valid, usable, acceptable, and able to sense most reported free-living activities, and provided feedback for improving the app. Design processes illustrate methodologic and interpretive efforts to operationalize motivational content in a theory-informed app promoting change in physical activity behavior.