Electrocatalytic nitrate reduction to ammonia represents a one-stone-two-birds approach for mitigating nitrate pollution and large-scale ammonia production. However, due to its multiple-electron process, it faces challenges regarding unsatisfactory Faraday efficiency (FE) and ammonia yield rate. Herein, a novel heterojunction electrocatalyst with sub-nanometric cobaltous oxide (CoO) decorated boron (B) nanosheets for electrocatalytic nitrate reduction (NO3RR) is first presented as an efficient electrocatalyst. The CoO/B heterojunction electrocatalyst exhibits a noteworthy NO3RR performance with an ammonia FE and yield rate of 95.41 % and 29.42 mg h−1 mgcat-1, respectively, in alkaline media at − 0.4 V vs. reversible hydrogen electrode (RHE), surpassing CoO and B catalysts. Results demonstrated that the synergistic effect of CoO and B enhances the adsorption and activation of NO3−, lowers the formation energy of *NOH and provides sufficient active hydrogen to facilitate hydrogenation, thereby enhancing NO3RR performance. This work presents a novel approach for constructing cost-effective and high-performance NO3RR heterojunction electrocatalysts.