Introduction Psychosis spectrum disorders are characterized by both positive and negative symptoms, but whereas there is good effect of treatment on positive symptoms, there is still a scarcity of effective interventions aimed at reducing negative symptoms. Rumination has been proposed as an important and fundamental factor in the development and maintenance of symptoms across psychiatric diagnoses, and there is a need to develop effective interventions targeting rumination behaviors and negative symptoms in patients with psychotic disorders. The aim of the current study is to investigate the feasibility and acceptability of group rumination-focused cognitive behavioral therapy (RFCBT) in the treatment of young people with psychosis spectrum disorders as well as investigating potential indications of treatment efficacy. Methods and analysis The study is a mixed-method clinical randomized controlled pilot trial with a target sample of 60 patients, who are randomized to either receive 13 weeks of group RFCBT or 13 weeks of treatment as usual (TAU). All patients are examined at the start of the project and at the 13-week follow-up. We will compare changes in outcomes from baseline to posttreatment between group RFCBT and TAU. In addition, qualitative analyzes are carried out to explore feasibility and acceptability and to uncover the patients’ experience of receiving the intervention.