Diabetes-induced oxidative stress is the key factor that initiates neuronal damage in the diabetic retina leading to diabetic retinopathy. This study was to investigate the possible effects of coumestrol (CMS) on inflammation, apoptosis and oxidative stress of retinal cells in streptozotocin-nicotinamide (STZ)-induced DR. Initially, a rat model of STZ was established by intraperitoneal injection of streptozotocin, and the expression of SIRT1 in retina tissues was determined by RT-qPCR. Next, the regulatory roles of CMS in oxidative stress, inflammation, and apoptosis of retinal cells were analyzed through detecting the expression of ROS, MDA, SOD, NO, iNOS, IL-6, TNF-α, and CRP, apoptotic factors (Cyt-C and Caspase-3) after ARPE-19 cells were treated with CMS. Moreover, the relationship between CMS and SIRT1 was analyzed. SIRT1 was lowly expressed in retina tissues. CMS treatment suppressed expression of ROS, MDA, iNOS, NO, IL-6, TNF-α, and CRP, as well as expression of Cyt-C and Caspase-3, but induced SOD expression in retina cells. CMS activated the expression of SIRT1, thereby inhibited oxidative stress, inflammation and apoptosis of retinal cells induced by DR. Taken together, the present study defines that CMS ameliorated DR by inhibiting inflammation, apoptosis and oxidative stress of retinal cells through the activation of SIRT1.