Background: Knee osteoarthritis (OA) is a leading cause of functional limitations among Veterans. Physical therapy (PT), with a focus on exercise, has strong evidence for improving physical function and other outcomes among individuals with knee OA. However, low adherence to home exercise is a major challenge both during the episode of care and following completion of therapy, significantly limiting the effectiveness of PT for knee OA. A recent systematic review highlighted the need for rigorous development and testing of interventions to enhance adherence to home exercise in the context of physical rehabilitation. In this project, we will test the effectiveness of a theoretically-informed, scalable intervention to enhance adherence to home exercise among Veterans receiving PT for knee OA. “Boosting Exercise Adherence in Knee Osteoarthritis” (BOOST-OA) includes components that address both exercise behavior initiation (conducted in conjunction with the PT episode of care) and maintenance (conducted after completion of PT care). Specifically, BOOST- OA includes: 1) Tools and activities woven into PT visits that address outcome expectations, action self- efficacy, goal-setting and monitoring (3-month behavior initiation phase) and 2) Health coaching calls that address satisfaction with outcomes, relapse prevention planning and independent monitoring (9-month behavior maintenance phase). Both phases will be supported by tailored messages through Annie, the VA’s text messaging service. The intervention will address strengthening and stretching exercises, as well as overall physical activity, as these are all part of a comprehensive approach to mitigating functional losses. Methods: We propose a pragmatic cluster-randomized trial, with 8 VA PT clinics randomized to BOOST-OA vs. usual PT care (UC). Participants will be Veterans with symptomatic knee OA (n=360, 45 per site). At BOOST- OA sites, physical therapy clinicians will deliver behavior initiation components in conj