The aim of this study is to evaluate the effectiveness of the nurse-led GILL (Gezondheid in Lichaam en Leefstijl) eHealth intervention in patients with serious mental illness (SMI), compared to usual care. Expected is that the GILL eHealth intervention will be more effective than usual care in improving physical health and lifestyle behaviors. To evaluate this, we will perform a cluster randomized controlled trial with an embedded process evaluation of the implementation of the GILL intervention. 258 adult patients with serious mental illness and a body mass index of 27 or higher (overweight/obesity) will be included. The GILL eHealth intervention consists of two complementary modules for (a) somatic screening and (b) lifestyle promotion, resulting in a personalized somatic treatment and lifestyle plan. Trained mental health nurses and clinical nurse specialists will implement the intervention within the multidisciplinary treatment context, and will guide and support the patients in the promotion of their somatic health, including cardiometabolic risk management. The intervention will be compared to usual care, which includes treatment according to national guidelines. The outcome measures will be metabolic syndrome severity (primary), fitness, physical activity, lifestyle behaviors, quality of life, recovery, psychosocial functioning, health related self-efficacy and health care utilization after 1 year. The process evaluation focuses on the feasibility of the eHealth intervention, its acceptability for patients and health care providers (mainly mental health nurses and clinical nurse specialists), and barriers/facilitators to implementation.