The aim of this study is twofold: firstly, to characterize the biochars resulting from the hydrothermal carbonization (HTC) of different types of wetland biomass and, secondly, to assess their potential for ammonia removal in aqueous solutions. The chars produced were carbon rich. The adsorption experiments indicated that hydrothermal treatment improved the ammonia removal efficiency rate, and loaded adsorbents were regenerable. Using an HTC char of reed as adsorbent has several advantages: it has the highest adsorption performance, the highest char yield, and the greatest affinity for regeneration.