Electrochemically converting nitrate, widely distributed in industrial wastewater and contaminated water bodies, into ammonia is a promising route for resource recovery and wastewater treatment. Meanwhile, treating harmful algal blooms (HABs) is presented worldwide, are time and resource-consuming, and carries a high CO2 footprint. Rather than considering this carbon and nitrogen-rich biomass as disposable waste, consider it a vast renewable resource. Therefore, this study presents a Fe-dispersed carbon-based catalyst derived from HABs biomass, with a maximum ammonia yield rate of 16449 μg h−1 cm−2 (1.2 mmol h−1 mgcat−1) and NH3 Faradaic efficiency of 87.3%. This catalyst also possessed decent stability with continuous operation over 50 h. Experimental and theoretical calculation results reveal that the Fe-N4 site facilitates electrocatalytic nitrate reduction reaction by reducing the energy barriers of the NO3–to-NH3 pathway. Thus, this strategy of upcycling HABs biomass waste into functional catalysts offers a multipronged approach to renewable and carbon-neutral energy technologies.