Background An increasing number of gastrointestinal cancer (GI) patients suffer from the side effects of cancer treatment that can affect their psychosocial health outcomes such as mood states and quality of life. Despite its demonstrated usefulness in female cancer patients, the Supportive Expressive Group Therapy (SEGT) has not been tested in male cancer patients. The current study sought to examine the longitudinal effects of a professionally-led, men-only supportive-expressive group therapy (SEGT) on mood states, coping, and quality of life (QoL) in male GI cancer patients.Methods Male GI cancer patients (n = 31), at different stages of cancerand its treatment, were recruited from an ongoing men-only biweekly GI cancer support group. Data were collected at baseline (before or near the beginning of group attendance) and at three months and six months follow-up. All outcomes were patient-reported and included demographic data as well as the validated questionnaires: Profile of Mood States (POMS) for mood states, Functional Assessment of Cancer Therapy-General (FACT-G) for QoL, and Ways of Coping-cancer version (WOC) for coping. Linear mixed models were used to examine the change in the outcomes over time. Effect sizes were estimated using Cohen’s d.Results The Anxiety (p = .04; d = 0.70), Depression (p = .03; d = 0.93) and Anger (p = .04; d = 1.28) subscales of the POMS decreased between baseline and six months. Participants also reported improvements in coping through Distancing (distancing oneself from negative thoughts, being more accepting of the situation, and looking for positives) of the WOC (p = .04; d = 0.4) between baseline and six months. There was no change in any of the FACT subscales (QoL) over time.Conclusions This is the first study to investigate the effects of a SEGT intervention in male cancer patients. SEGT resulted in improved mood states and coping in male GI cancer patients; however, there was no change in measures of QoL. Given the direct effects of SEGT on anxiety, depression and anger, as well as a strong association between anger and depression in men as reported in past studies, SEGT could be impactful in improving psychosocial health outcomes in male GI cancer patients.