AIMS: Diabetes is associated with poorer prognosis and treatment-related outcomes in patients with cancer. Major surgical resection is the mainstay of potentially curative treatment for colorectal cancer (CRC). This study aimed to assess whether the risk of adverse outcomes for individuals with diabetes and CRC varies by diabetes status and associated diabetes-related complications. METHODS: Information for all individuals diagnosed with CRC in England between 2011 and 2021 was obtained from cancer registry data. Pre-existing diabetes was identified using diagnostic codes during hospital inpatient stay. Cox regression and logistic regression were used to assess the relationship between diabetes complication status and postoperative outcomes (5-year survival, 90-day mortality, death in hospital and unplanned readmission). RESULTS: Of the 372,477 individuals diagnosed with CRC, treatment using major surgical resection was highest amongst those with no diabetes (60%) and diabetes without complications (62%) compared to those with combined (microvascular and macrovascular) complications (34%). Five-year survival was lowest for those with combined complications when compared to those with no diabetes (45% vs. 69% after major resection; 5% vs. 18% without major resection). Increasing levels of complication severity were associated with increasing rates of 90-day postoperative mortality, with combined complications associated with the poorest outcome when compared to those without diabetes (10% vs. 4%, adjusted OR 2.18, 95% CI 1.90-2.51). CONCLUSIONS: This population-based study demonstrates that the risk of adverse outcomes in patients with diabetes and CRC is heterogeneous. Future work is required to understand whether postoperative outcomes can be improved in individuals with diabetes and CRC.