Many low income countries are experiencing a “nutrition transition” towards the consumption of more energy-dense, highly processed foods and beverages that are often high in caloric sweeteners, fat and salt. Changing lifestyles and urbanisation have coincided with a ‘retail revolution’, a rapid advance of supermarkets even in remote areas. Among the consequences of the nutrition transition have been expanding waistlines and surging rates of nutrition-related non-communicable diseases, including diabetes, heart diseases and certain cancers. Given the still prevailing rates of under-nutrition, affected countries face a double burden of malnutrition, and individuals that have overcome food poverty risk often remain health-poor. The effect of supermarkets on consumers’ diets and the nutrition transition remains unclear: By offering stable and consistent access to a wide range of foods with different dietary qualities, supermarkets could either discourage or contribute to the consumption of a well-balanced diet. This paper investigates the effect of supermarkets on consumption patterns using cross-sectional household survey data collected in Kenya in 2012. In order to establish causality, our sample was designed to be quasiexperimental in nature, with study sites differing in terms of supermarket access. We employ instrumental variable techniques to account for potential endogeneity due to selection effects regarding supermarket purchases. Our findings suggest that supermarket purchases increase the consumption of processed foods at the expense of unprocessed foods. Supermarkets are associated with higher expenditure shares and calorie shares of processed foods, and with increased per capita calorie availability. The latter effect is supported by lower prices per calorie for processed food items. Supermarket purchases have a positive effect on dietary diversity, but implications for the nutrient adequacy of consumers remain unclear.