Cancer patients diagnosed with type 2 diabetes mellitus (T2DM) are at an increased risk of death due to cancer. However, whether T2DM comorbidity increases other causes of death in cancer patients is the novel theme of this study. Patients with T2DM were identified from the nationwide Swedish Hospital Discharge Register and linked with patients with cancer recorded from the Swedish Cancer Registry. Hazard ratios (HRs) were calculated for death due to all causes among cancer patients with and without T2DM; both underlying and multiple causes of death were examined using the Cox regression model. A total of 13 325 cancer patients were identified with comorbidity of T2DM. The total number of deaths of cancer patients was 276 021. Of these, 5900 occurred after T2DM diagnosis. For underlying causes of death, except for T2DM, the highest cause-specific HRs were found for complications of bacterial disease (HR, 3.93; 95% CI, 3.04–5.09), urinary system disease (HR, 3.39; 95% CI, 2.78–4.12), and myocardial infarction (HR, 2.93; 95% CI, 2.75–3.12). When risk of death was examined for both underlying and multiple causes of death, the highest HRs were found for hypertensive disease (HR, 3.42; 95% CI, 3.15–3.72), urinary system disease (HR, 3.39; 95% CI, 3.17–3.63), and arterial disease (HR, 3.26; 95% CI, 3.08–3.46). The diagnosis of T2DM in cancer patients is associated with an increased risk of death due to various causes, including myocardial infarction, other bacterial disease, urinary system disease, hypertensive disease, arterial disease, and so on, which may be related to both cancer and treatment. Clinicians that treat cancer patients with T2DM should pay more attention to comorbidities.