Child marriage is a global health and human rights issue. In Cameroon, 30% of women are married before age 18 but little research exists on the drivers of child marriage in the country. This qualitative study contributes to understanding the role of social norms in sustaining child marriage in Far-North and East Cameroon. Participants in the study (N = 80) included women and men from four, ethnically different, rural communities (two in the Far-North, two in the East). Methods for data collection included 16 semi-structured focus groups, in which we investigated the system of social norms sustaining child marriage in these communities. We asked participants about typical age at marriage for girls (local practices) and whether they believed that age to be appropriate (their attitudes). We found the relation between practices and attitudes to be different in each community. We discuss the implications of these different relations for social norms interventions, enriching existing theoretical explanations. Evidence emerging from our findings suggest that effective social norms interventions should be embedded within cultural understandings of the relations between people's attitudes and practices.