Objective: A health action process approach (HAPA)-based intervention was designed to support moderate-to-vigorous physical activity (MVPA) in individuals with osteoarthritis of the knee (OAK). In secondary analyses of the randomized controlled trial “preventing the impairment of primary osteoarthritis by high-impact long-term physical exercise regimen—psychological adherence program,” we examined long-term effects of the intervention on HAPA determinants and MVPA, and explored the former as mediators of change. Method: N = 241 individuals with OAK (63% women, aged 44–80 years) were randomly assigned to the 12-month intervention condition (IC) or active control condition (CC). Between 2016 and 2020, self-reported HAPA determinants (action and coping planning, maintenance and recovery self-efficacy, action control) and collaborative planning were assessed at 0, 6, 12, 18, and 24 months, accelerometer-assessed MVPA at 0, 12, and 24 months. Multilevel and manifest path models were fit. Results: Compared to the CC, action planning was higher in the IC at 6, 12, and 24 months. Maintenance and recovery self-efficacy were stable in the IC but decreased in the CC. MVPA decreased in both conditions. More action planning in the IC at 12 months was related to higher MVPA at 24 months, but, as in all other models, the indirect effect was nonsignificant. Conclusions: The intervention partly stabilized or enhanced HAPA determinants but did not increase MVPA in a Western, highly-educated sample with OAK. Future work might use blended-care approaches enriched by mobile applications for continuous MVPA support. (PsycInfo Database Record (c) 2025 APA, all rights reserved) Impact Statement: This theory-based intervention was developed to support individuals with knee osteoarthritis in long-term physical activity. Whereas the intervention was partly successful in enhancing or stabilizing psychological predictors, it did not increase or stabilize physical activity over 2 years. Future research may further refine the intervention by adding digital applications to provide continuous physical activity support. (PsycInfo Database Record (c) 2025 APA, all rights reserved)