Chronic musculoskeletal diseases (MSD) are a leading cause of disability, and outdoor physical activity (PA) has been proposed as a setting that may facilitate PA engagement. To report immediate changes in health-related quality of life (HRQOL), pain, physical function, physical activity, and mental well-being following an outdoor PA intervention in people with MSD, and to explore whether changes vary depending on the underlying MSD. This prospective cohort study included 925 adults (mean age: 65 years; 93% female) with MSD who participated in a 12-week, once weekly, 60-min outdoor group-based PA program in Denmark. Sessions were led by trained volunteer instructors and consisted of PA, complemented by mental exercises in natural environments, including parks and urban areas. Pre- and immediately post-intervention data were collected using questionnaires and performance-based physical tests. Outcomes included EQ-5D-5L (HRQOL), visual analogue scale (pain), WHO-5 (mental well-being), UCLA activity score (physical activity), and 40-meter walk and 30-second chair stand tests (physical function). Linear mixed-effect models were used to assess changes and compare subgroups. In the total cohort, significant improvement was observed only in HRQOL (mean score change: 0.01, P = 0.003). In a subsample with performance-based physical test data (n = 361), participants improved in walking speed (mean score change: +0.13 m/s, P ˂ 0.001) and chair stands (mean score change: +2.9 repetitions, P ˂ 0.001). Participants with inflammatory or degenerative spine-related diseases tended to report less favorable outcomes post-intervention than participants without these diseases. The OUTPAC study found no mean improvement in most self-reported outcomes, aside from small, significant improvements in HRQOL from the low-dose outdoor PA intervention. Performance-based physical tests showed significant improvements in a subsample, and higher attendance was associated with better HRQOL and mental well-being. Secondarily, participants with degenerative or spine-related diseases responded less favorably. Future research should improve data collection, increase exercise dose, and explore differences across MSD subtypes through high-quality randomized controlled trials. Danish Data Protection Agency (REG-147-2021).