Objectives. Limited research exists addressing the development of health-related smartphone apps, a new and potentially effective health promotion delivery strategy. This article describes the development and formative evaluation of a smartphone app associated with a physical activity promotion website. Methods. A combination of qualitative and quantitative techniques (performance measures, direct observation, and subjective participant preferences) were implemented during two usability testing sessions (pre- and postmodification) while participants were completing tasks using the app. Results. Design improvements to the app resulted in a reduction in the problems experienced and a decrease in the time taken to complete tasks. Four usability themes emerged from the data: design, feedback, navigation, and terminology. Conclusion. This study demonstrates the relevance of usability testing to the design and modification of a smartphone app related to a health promotion website. This study resulted in an app with much higher usability, which might increase usage and maintenance of health behavior change in the long term. Practical implications. This study demonstrates the need for formative evaluation in health-related smartphone apps. Attention should be given to basic design principles as well as feedback, navigation, and terminology in order to ensure utility and ease of use of future smartphone app designs.