Patients with severe mental disorders die on average 20 years prior to the general population. This mortality gap is mainly due to the higher prevalence of physical diseases and the adoption of unhealthy lifestyle behaviors.The LIFESTYLE trial aims to evaluate the efficacy of a new psychosocial group intervention (including psychoeducational, motivational, and problem-solving techniques) focused on healthy lifestyle behavior compared to a brief educational group intervention in a community sample of patients with severe mental disorders. The trial is a national-funded, multicentric, randomized controlled trial with blinded outcome assessments, which is carried out in six outpatient units of the Universities of Campania “Luigi Vanvitelli” in Naples, Bari, Genova, L'Aquila, Pisa, and Rome—Tor Vergata. All patients are assessed at the following time points: baseline (T0); 2 months post-randomization (T1); 4 months post-randomization (T2); 6 months post-randomization (T3); 12 months post-randomization (T4); and 24 months post-randomization (T5). T1 and T2 assessments include only anthropometric tests. The BMI, a reliable and feasible anthropometric parameter, has been selected as primary outcome. In particular, the mean value of BMI at 6 months from baseline (T3) will be evaluated through a Generalized Estimated Equation model. The work hypothesis is that the LIFESTYLE psychosocial group intervention will be more effective than the brief educational group intervention in reducing the BMI. We expect a mean difference between the two groups of at least one point (and standard deviation of two points) at BMI. Secondary outcomes are: the improvement in dietary patterns, in smoking habits, in sleeping habits, physical activity, personal and social functioning, severity of physical comorbidities, and adherence to medications. The expected sample size consists of 420 patients (70 patients for each of the six participating centers), and they are allocated with a 1:1 ratio randomization, stratified according to center, age, gender, and educational level. Heavy smoking, sedentary behavior, and unhealthy diet pattern are very frequent and are associated with a reduced life expectancy and higher levels of physical comorbidities in people with severe mental disorders. New interventions are needed and we hope that the LIFESTYLE protocol will help to fill this gap.Trial registration number: 2015C7374S.