‘Complexity’ is theorised as a characteristic of modern food systems that poses a challenge to improving population diets. This paper seeks to explore the discursive deployment of ‘complexity’ in the context of public health. Doing so helps inform a more critical assessment of commercial and political determinants of health, and of ‘complexity’ as a prevailing frame for public health issues. Qualitative methods were used to explore ‘responsibility’ for healthy eating in the food system in the United Kingdom. Discourse analysis was conducted on corporate and government documents, and interviews with industry and government stakeholders. The aim was to examine the implications of ‘complexity’ within discourses of dietary public health. ‘Complexity’ was used not only to characterise dietary public health problems but also as a rhetorical device in public health narratives. It performed two main discursive functions: firstly, to represent diet-health problems as so multi-layered and difficult that they are intractable. Secondly, and despite this acknowledged complexity, to deflect from food system actions for improving diet to ‘simpler’ and non-food interventions, by industry and government. These uses of ‘complexity’ serve to attribute primary responsibility to individuals for dietary choice and to obscure inaction by government and industry, who have most control over the determinants of those choices. In short, ‘complexity’ can be used discursively to generate a smokescreen masking policy inaction in addressing public health problems.