Interpersonal distress is a common feature in gambling disorder and adding a concerned significant other (CSO) to the recovery process could be an effective tool for improving treatment outcome. However, little empirical evidence is available regarding the effectiveness of including a CSO to interventions. We aimed to compare treatment outcomes (i.e. compliance with therapy guidelines, dropout from treatment, and relapse during treatment) in a CBT program involving a CSO to CBT treatment as usual (TAU) without a CSO. The sample comprised male gambling disorder patients (N = 675). The manualized CBT intervention consisted of 16 weekly outpatient group sessions and a 3-month follow-up period. Patient CSOs attended a predetermined number of sessions with the patient and were provided with resources to acquire a better understanding of the disorder, to manage risk situations, and to aid patients in adhering to treatment guidelines. Patients with a CSO had significant higher treatment attendance and reduced dropout compared to patients receiving TAU. Moreover, patients whose spouse was involved in the treatment program were less likely to relapse and adhered to the treatment guidelines more than those with a non-spousal CSO. Our results suggest that incorporating interpersonal support to gambling disorder interventions could potentially improve treatment outcomes.