Maternal grandmothers residing in 3-generation households often provide care and support to their grandchildren. However, the implications of grandmother coresidence and involvement for adolescent adjustment have been neglected in the South African literature. This study examined whether the involvement of maternal grandmothers who coreside with grandchildren and their parents differed from that of non-coresident grandmothers. In addition, we assessed the associations between maternal grandmother coresidence and involvement, and adolescents’ internalising problems, externalising problems and prosocial behaviour. Self-report survey data were obtained from a sample of 384 ‘coloured’ (mixed-race) and black African Grade 8 and Grade 9 students in Cape Town. The mean age of the participants was 13.96 years, 58% were females and 27% lived in 3-generation households. Results indicated that there was no significant difference in the involvement of coresident and non-coresident grandmothers, and that adolescents in 3-generation and 2-generation households displayed similar levels of adjustment. Hierarchical multiple regression analyses revealed that greater maternal grandmother involvement was associated with more adolescent prosocial behaviour (p < .001) regardless of household structure, and with fewer adolescent internalising problems in 3-generation households (p = .03). Findings underscore the need to move beyond the immediate family to consider how grandparents may influence adolescent development.