The prevalence of diabetes reached 415 million worldwide in 2015. Polyphenol-rich food intake can benefit the glycemic control for individuals with diabetes. Fermentation may increase the bioavailability of polyphenols, which is generally low. Aronia (Aronia melanocarpa) is a polyphenol-rich berry that is native to North America. Proanthocyanins and anthocyanins are the major phenolic compounds in aronia. In this study, aronia kefir was made by fermenting cow's milk with added aronia juice. The changes in bioaccessible polyphenols of aronia kefir during digestion were assessed using an in vitro model. The impact of fermentation on the potential bioactivity of aronia polyphenols was evaluated. Results showed that the bioaccessible polyphenols in aronia kefir were elevated during digestion and the antioxidant capacity increased (IC50 of DPPH scavenging decreased from 24.07 mg kefir per mL to 8.97 mg kefir per mL). Digested aronia kefir had less bioaccessible anthocyanins (cyanidin-3-galactoside, cyanidin-3-arabinoside and cyanidin-3-xyloside) but similar antioxidant capacity and stronger inhibitory activity on α-glucosidase (IC50: 152.53 mg kefir per mL) compared to the non-fermented control (IC50: 484.93 mg kefir per mL). These results indicate that fermentation may produce metabolites with higher antioxidant capacity and better α-glucosidase inhibitory activity. Utilizing aronia kefir in the diet is a good strategy to help control blood glucose levels without abdominal side effects. Fermentation may be an effective method to increase the bioavailability of dietary polyphenols in food. More studies about the effects of fermentation on polyphenol-rich food are needed to optimize the potential health-promoting properties.