Presently there is a lack of evidence regarding the impact of gender composition of mixed-gender weight management therapy groups on members’ posttreatment outcomes. As such, the current study tested the association between the gender composition of mixed-gender narrative therapy groups for weight management and members’ posttreatment outcomes. Specifically, we examined the variability in group members’ adjusted posttreatment psychological (Outcome Questionnaire-45 [OQ-45]) and obesity-related (Obesity Related Well-Being [ORWELL 97]) symptoms as a function of the quadratic proportion of female members in a group, and the interaction between group member gender and the quadratic proportion of female group members. Pre- and posttreatment OQ-45 and ORWELL 97 data was collected from 32 men and 67 women participating in 15 short-term weight-management therapy groups. Results indicated that the quadratic proportion of female group members was significantly related to adjusted posttreatment OQ-45 scores, such that group members reported fewer psychological symptoms at posttreatment in more proportionally balanced groups of men and women. This relationship was not significant for adjusted posttreatment ORWELL 97 scores. Additionally, in an exploratory analysis we found a significant interaction between group member gender, the proportion of female group members, and group members’ adjusted posttreatment OQ-45 scores, indicating that the relationship between gender composition and adjusted posttreatment OQ-45 scores was stronger for men than women. These preliminary findings suggest that the gender composition of weight management groups is an important factor in group members’ posttreatment psychological outcomes.