While food insecurity is a well-known cause of under-nutrition and stunting, in recent decades it has also been linked with obesity. Understanding and thus minimising the risk factors for obesity in low- and middle-income country contexts, which often lack the health system capacity to treat the consequent obesity-related illnesses, is crucial. This study adopted realist review methodology because it enabled us to analyse and organise the evidence from low- and middle-income countries into a coherent scheme. By comparing this evidence to existing theory on food security and nutrition, we aimed to provide a richer understanding of the nuances and the socio-demographic nature of the food insecurity-obesity relationship. The review generated 13 peer-reviewed articles which studied the relationship between food insecurity and overweight/obesity in low- and middle-income countries. Affordability of high-energy, processed foods was identified as a main mechanism, which determined whether or not food insecurity leads to obesity in low- and middle-income countries. Other mechanisms identified were: quantity and diversity of food consumed; spatial-temporal access to nutritious food; inter-personal food choice and distribution; and non-dietary behaviours. Despite the limited empirical evidence available, our review presents meaningful and policy-relevant insights into the food insecurity-obesity relationship in from low- and middle-income countries. Interventions to address the food insecurity-obesity link need to address diet quality, and demand a broad understanding across a variety of experiences.