Objective: Functional impairments can lower psychological well-being after brain tumor. Changes in social groups and confidence in support potentially impact this relationship. This study aimed to investigate the influence of social group memberships (SGMs) on the relationship between perceived cognitive and physical impairment and psychological well-being. Methods: Seventy adults (60% female) with primary brain tumor (46% benign; 18% low grade; 36% high grade) aged 22 to 75 years undertook a brief cognitive test (Brief Test of Adult Cognition by Telephone) then completed self-report measures of cognitive and physical impairment (Functional Assessment of Cancer Therapy), social groups (Exeter Identity Transition Scale), confidence in social support (Self-Efficacy Scale), depression (depression scale of the 21-item Depression, Anxiety and Stress Scale), anxiety (General Anxiety Disorder 7-item scale), and life satisfaction (Satisfaction With Life Scale). The mediating and moderating effects of SGMs were tested using a bootstrapping method and PROCESS macro. Results: Greater perceived cognitive and physical impairments were significantly related to poorer psychological well-being. Mediation analyses indicated significant indirect effects of social group loss for depression and anxiety (P <.05), whereby the relationship between perceived functional impairment and depression and anxiety was partly accounted for by loss of SGMs. Confidence in social groups was a moderator for depression and anxiety (P <.001), such that those perceiving high levels of physical and cognitive impairments who were more confident in their social groups reported lower depression and anxiety. Conclusions: Functional impairment is in part related to higher levels of depression and anxiety through loss of social groups. However, greater confidence in social support can buffer the effects of functional impairment on psychological well-being after brain tumor. Interventions focusing on ways to maintain social participation and supportive relationships may be beneficial.