This paper reports on a qualitative study on social norms and child marriage in rural Cameroon, a country with high prevalence of child marriage but largely ignored in the literature. Study participants (n = 80) were men and women from four different ethnic groups living in four rural villages (two in the Far-North, two in the East). With the assistance of four local interviewers, we conducted 16 semi-structured focus groups to understand how existing social norms contributed to child marriage in participants' communities. We found great variety in the influence of social norms on people's health-related practices: across these four communities, social norms made compliance with the child marriage practice (respectively) possible, tolerated, appropriate, and obligatory. Effective health promotion interventions should be grounded within sound theoretical understandings of the varying influence of social norms. Using data on child marriage, this paper offers a case study of how that understanding can be developed.