Background: Harmful alcohol use is a public health problem associated with negative health and socio-economic impacts. However, patterns and dynamics of alcohol use among slum-dwellers in Kenya are poorly understood.Objective: To establish determinants of harmful alcohol use among adults in an urban slum setting in Kenya.Materials and methods: Cross-sectional study involving a consecutively selected sample (N=215) from Githurai, in Nairobi. A pre-tested questionnaire that captured data on socio-demographics, drinking patterns, type, reasons, initiator, and support system.Results: Of the respondents, those above 31 years, married, separated/divorced/widowed, of high education, earning above 50 USD, and from dysfunctional families consumed more alcohol. Low earners consumed (p < 0.05) unrecorded while high earners drank (p< 0.001) recorded alcohol. Adults from families with a drinking father and sibling consumed more alcohol (p=0.001). Single, low educational attainment/earners, and those in dysfunctional families (p <0.05) drank due to stress and reported alcohol-related problems. Young, unmarried, and casual laborers were introduced (p < 0.05) to alcohol by friends.Conclusion: Socio-demographic, economic, familial, social interactions, and stress are associated with harmful alcohol use among adults from slums calling for interventions targeting these factors.Keywords: Determinants; urban slum dwelling; alcohol use; alcohol abuse; adults; informal.