Background Approximately 60,000 people in England have coexisting type 2 diabetes mellitus (T2DM) and severe mental illness (SMI). They are more likely to have poorer health outcomes and require more complex care pathways compared to those with T2DM alone. Despite increasing prevalence, little is known about the healthcare resource use and costs for people with both conditions. Aims To assess the impact of SMI on healthcare resource use and service costs for adults with T2DM, and explore the predictors of healthcare costs and lifetime costs for people with both conditions. Method Matched cohort study using data from Clinical Practice Research Datalink (CPRD) linked to Hospital Episode Statistics (HES) for 1,620 people with comorbid SMI and T2DM and 4,763 people with T2DM alone. Generalised linear models (GLMs) and the Bang and Tsiatis method were used to explore cost predictors and mean lifetime costs respectively. Results People with T2DM and SMI had higher average annual costs (£1,930) than people with T2DM alone, driven primarily by mental health and non-mental health-related hospitalisations. Key predictors of higher total costs were older age, comorbid hypertension, use of antidepressants and first-generation antipsychotics, and increased duration of living with both conditions. Expected lifetime costs were approximately £35,000 per person with both SMI and T2DM. Extrapolating nationally, this would generate total annual costs to the NHS of around £250m per year. Conclusions Our estimates of resource use and costs for people with both T2DM and SMI will aid policy makers and commissioners in service planning and resource allocation.