Greater body mass index (BMI) is increasingly recognised as a risk factor for the development of dementia. This study investigated the functional connectivity networks associated with BMI using Network-Based Statistics in a cohort at-risk of developing dementia. Greater BMI was associated with decreased connectivity within 3 networks. Two of these networks showed decreased thalamo-cortical coupling and the third showed reduced connectivity between two nodes in the frontal cortex. This study illustrates the influence of BMI on functional connectivity, and a potential mechanism through which higher BMI confers risk of conversion from an at-risk clinical state to a dementia diagnosis.