Sodium borohydride (NaBH) is among the most studied hydrogen storage materials because it is able to deliver high-purity H at room temperature with controllable kinetics via hydrolysis; however, its regeneration from the hydrolytic product has been challenging. Now, a facile method is reported to regenerate NaBH with high yield and low costs. The hydrolytic product NaBO in aqueous solution reacts with CO, forming NaBO⋅10 HO and NaCO, both of which are ball-milled with Mg under ambient conditions to form NaBH in high yield (close to 80 %). Compared with previous studies, this approach avoids expensive reducing agents such as MgH, bypasses the energy-intensive dehydration procedure to remove water from NaBO⋅10 HO, and does not require high-pressure H gas, therefore leading to much reduced costs. This method is expected to effectively close the loop of NaBH regeneration and hydrolysis, enabling a wide deployment of NaBH for hydrogen storage.