Abstract Background Forensic psychiatric patients often suffer from a multitude of severe psychiatric and social problems. Meanwhile multimodal evidence-based interventions are scarce and treatment effectiveness is in need of improvement. The main goal of forensic psychiatric treatment is to address psychiatric and social factors and thereby mitigate criminal behaviour. Notably, a supportive social network is an important protective factor for criminal behaviour. As such, improving a poor social network may decrease the risk of criminal recidivism. This study aims to examine the effectiveness of the addition of an informal social network intervention (FNC) to treatment as usual (TAU) among forensic psychiatric outpatients. Methods In a mono-center randomized controlled clinical trial with two parallel groups, forensic psychiatric outpatients with social network-related problems (N = 105) will be allocated to either TAU + FNC or TAU alone. The informal social network intervention consists of a 12-month coaching intervention, performed by the forensic network coach (a volunteer trained by an informal care institute). Assessments will be conducted at baseline and 3 months, 6 months, 9 months, 12 months, and 18 months after baseline. The primary outcome variable is mental wellbeing. Psychiatric functioning, criminal recidivism, substance abuse, quality of life, social network, social support, loneliness and self-sufficiency are included as secondary outcomes. A variety of potential mediators and moderators of effectiveness will be explored. Additionally, a qualitative evaluation of effectiveness will be performed. Discussion This study will contribute to the existing literature of forensic treatment effectiveness as it is the first RCT examining the effectiveness of adding a social network intervention in a forensic outpatient population. If effectiveness is shown, forensic mental health care could be optimized by collaborating with informal care or community initiatives aimed at improving a positive social network. In addition, results will provide insight regarding mediators and moderators of treatment effectiveness. Trial registration This study is registered at the Netherlands Trial Register (NTR7163). Date of registration: 16 april 2018.