There is a globally increasing prevalence and incidence of diabetes mellitus (DM). Prolonged hyperglycaemia could lead to both macrovascular damage, such as carotid artery atherosclerosis, and microvascular damage, such as retinal arteriolar narrowing, and might contribute to retinal artery occlusion (RAO). Accordingly, it is important to determine whether DM is a contrubuting factor of RAO. We conducted a retrospective cohort study that included 241,196 DM patients from the Longitudinal Cohort of Diabetes Patients Database who were recruited between 2003 and 2005. An age- and sex-matched non-DM control group included the same number of patients who were selected from the Taiwan Longitudinal Health Insurance Database of 2000. Relevant data of each patient were collected from the index date until December 2013. The incidence and risk of RAO were calculated and compared between the DM and non-DM groups. The hazard ratio for RAO was calculated using Cox proportional hazards regression analysis after adjusting for confounders. The cumulative incidence rate of RAO was calculated by Kaplan-Meier analysis. In total, 317 patients with DM and 144 controls developed RAO during the follow-up period, leading to an incidence rate of RAO in DM patients that was 2.30 times (95% confidence interval [CI] = 1.89-2.80) greater than that in controls. After adjustment for potential confounders, patients with DM were 2.11 times (95% CI, 1.71-2.59) more likely to develop RAO in the total study cohort. In conclusion, DM increases the risk of RAO, which is an interdisciplinary emergency. Close collaboration between endocrinologists and ophthalmologists is important in managing RAO following DM.