Electrocatalytic reduction of nitrite (NO₂⁻) is considered an effective method to solve its environmental pollution issue as well as sustainable ammonia (NH₃) synthesis under ambient conditions but still requires high-performance catalysts. Herein, self-supported monoclinic WO₂ nanoparticles with abundant oxygen vacancies (OVs) on a tungsten plate (WO₂/W) are demonstrated as a highly efficient electrocatalyst for selectively reducing NO₂⁻ to NH₃. In 0.1 M NaOH solution containing 0.1 M NO₂⁻, such an electrocatalyst achieves an excellent yield of 14 964.25 ± 826.06 μg h⁻¹ cm⁻² and high faradaic efficiency (FE) of 94.32 ± 1.15% at −0.9 V. Remarkably, the assembled Zn–NO₂⁻ battery delivers a high power density of 5.05 mW cm⁻² and simultaneously offers an abundant NH₃ yield of 1923.88 μg h⁻¹ cm⁻². Theoretical calculations reveal the critical role of OVs for WO₂ in the nitrite electrocatalytic reduction process and its possible reaction pathway.