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Abstract Background People with severe mental illness (SMI) face not only impaired mental health, but also a greater risk of physical comorbidities and a shorter life expectancy compared to the general population. A poor lifestyle plays a substantial role in this disparity. Combined Lifestyle Interventions targeting multiple lifestyle behaviors can improve mental and physical health, and quality of life. However, there is currently no appropriate structural support for people with SMI in outpatient care in the Netherlands. The Combined Lifestyle Intervention for Outpatients with SMI (GOAL!) is developed to address this gap. This study examines the (cost-)effectiveness and implementation of GOAL!. Methods In a type 1 hybrid quasi-experimental study with a mixed-method matched design, GOAL! participants (N = 50) are compared to people receiving care as usual (N = 50). The GOAL! program includes group and individual sessions, given by allied health professionals, over a period of two years. The first year starts with a 3-month intensive course on physical activity and nutrition, followed by 9 months of aftercare covering various lifestyle topics tailored to the group’s needs. There is close collaboration with local stakeholders to facilitate transfer to the community setting. The second year focuses on maintaining established activities in one’s daily living environment. Our primary outcome will be the change in physical activity, comparing GOAL! participants to those receiving care as usual. Secondary outcomes are changes in other lifestyle behaviors, physical health, mental well-being, and healthcare and societal costs. Additionally, achieving lifestyle-related goals, adverse effects, and barriers and facilitators to implementation are examined. Measurements are obtained at start (T0), and after 3 (T1), 12 (T2) and 24 months (T3). Discussion This study investigates the effects of GOAL! on lifestyle behaviors, health outcomes, implementation factors and cost-effectiveness after two years, aiming to offer valuable insights into the effectiveness and implementation outcomes of lifestyle interventions for outpatients with SMI. Trial registration : ClinicalTrials.gov (Identifier: NCT05600205). Prospectively registered on October 26, 2022.