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Objectives To compare influenza-related hospitalisation, mortality and effects of background factors in adults with type 2 diabetes (T2D) and age-matched and sex-matched controls. To explore associations between clinical variables and the risk of severe influenza outcomes.Design Register-based nationwide cohort study.Settings Data from the National Diabetes Register are cross-linked to the Swedish Population Register, Statistics Sweden and the Swedish Patient Register.Participants 371 811 patients with T2D from the Swedish National Diabetes Register and 1 728 856 matched control individuals from the Swedish population followed over the 2013/14–2018/19 influenza seasons.Primary and secondary outcomes Data on hospital admissions, mortality, socioeconomic factors and pre-existing conditions were collected. Risk associations were analysed using Cox proportional hazards models. Within the diabetes group, non-linear associations between common clinical variables and influenza hospitalisation were examined.Results 1.6% of those with diabetes and 1.0% of controls were hospitalised for influenza, with influenza-related mortality at 0.12% and 0.08% respectively. Adjusted HR for hospitalisation in T2D was 1.57 (95% CI 1.52 to 1.61) and 1.44 (95% CI 1.29 to 1.61) for mortality. Absolute risk was highest in those with cardiovascular, kidney or respiratory disease. Relative risk was greatest in younger (<65 years) patients with T2D. Higher haemoglobin A1c (HbA1c) and lower estimated glomerular filtration rate (eGFR) were linked to increased hospitalisation risk in persons with T2D.Conclusion This study confirms that T2D increases the risk of hospitalisation and mortality from seasonal influenza. Support to achieve HbA1c and eGFR targets and following vaccination guidelines is important.