This study used Rasch analysis to examine the psychometric validity of the Illness Perception Questionnaire–Revised to assess beliefs about diabetes in 470 participants with Type 2 diabetes and 71 participants with Type 1 diabetes. All Illness Perception Questionnaire–Revised scales had psychometric issues, which included poorly utilised response categories, poor scale precision and multidimensionality. Following re-engineering, only four of the eight scales (Consequences, Illness coherence, Timeline cyclical and Emotional representations) were psychometrically adequate according to the Rasch model. The diabetes-specific version of the Illness Perception Questionnaire–Revised provides suboptimal assessment of beliefs held by patients with diabetes.