Background: Long-term complications of diabetes include cardiovascular disease, retinopathy, nephropathy, and neuropathy. Diabetic patients with prostate cancer could be at a high risk of radiation-induced acute proctitis following radical radiotherapy. Our aims were to analyse the incidence, severity, and duration of radiation proctitis in diabetic patients treated by radical radiotherapy and combined androgen deprivation for prostate cancer. Material and methods: On the bases of inclusion and exclusion criteria 716 patients with prostate cancer were retrospectively recruited. Patients were stratified into diabetic patients and non-diabetic patients. The incidence, severity, and duration of proctitis were the main outcomes. A polynomial ordered logistic regression was fitted to determine the influence of diabetes status, age, blood pressures medication, co-morbidities, Gleason score, PSA after treatment, and tumour stage on the grades of proctitis. Time to resolution per year was modelled as a negative binomial generalised linear model. Results: The overall mean age of patients was 67.44 (SD 6.77) years with a follow-up time of 3.36 (SD 2.05) years. Data exploratory analysis suggested that the only highly significant explanatory variable was the presence or absence of diabetes. Polynomial ordered logistic regression, however, showed that the presence (or not) of diabetes remained as the only significant predictor (t = −2.74; p = 0.0059) of severity of proctitis. A negative binomial generalised linear model showed that both grade of proctitis (z = −17.178; p < 0.001), and diabetes (z = −5.92; p < 0.001), were highly significant predictors of time to resolution. Conclusions: Diabetic patients were significantly more likely to have proctitis after radical radiation therapy for prostate cancer. Diabetes was significantly associated with an induced risk of radiation induced proctitis and also with deceleration of its resolution.