PURPOSE: Smartphone-based interventions offer a promising approach to address inactivity in people with knee osteoarthritis (OA). We explored perceptions towards smartphone-based interventions to improve physical activity, pain, and depressed mood in inactive people with knee pain. METHODS: This qualitative study included six focus groups at Boston University with inactive people with knee pain ( = 35). A smartphone app, developed by our team, using constructs of Social Cognitive Theory, was used to obtain participant feedback. RESULTS: Participants discussed wanting to use smartphone-based interventions for personalized exercise advice, for motivation (e.g., customized voice messages, virtual incentives), and to make exercise "less boring" (e.g., music, virtual gaming). Preferred app features included video tutorials on how to use the app, the ability to select information that can be viewed on the home screen, and the ability to interact with clinicians. Features that received mixed responses included daily pain tracking, daily exercise reminders, peer-interaction for accountability, and peer-competition for motivation. All participants discussed privacy and health data security concerns while using the app. CONCLUSIONS: Using a co-design approach, we report preferences and concerns related to using smartphone-based physical activity interventions in inactive people with knee pain. This information may help improve acceptability of such interventions in this population.