The sulfidation of metallic silver nanoparticles (AgNP) observed in urban wastewater systems and in natural waters reduces their toxicity by several orders of magnitude. However, the reaction rate of this transformation is only poorly understood and the influence of humic acid (HA) on AgNP sulfidation has not been studied to date. We therefore investigate the sulfidation kinetics of AgNP reacted with bisulfide (HS⁻) in the absence and presence of HA and evaluate different kinetic models to describe the observed reaction kinetics. Citrate-stabilized AgNP of different sizes (20–200 nm) were reacted with an excess of HS⁻ in the absence of HA as well as at HA concentrations ranging from 50 to 1000 mg L⁻¹. The extent of AgNP sulfidation after the selected reaction times was determined by X-ray absorption spectroscopy (XAS). The overall sulfidation rate increased with decreasing AgNP size and increasing HA concentration. The sulfidation rate of the AgNP was best described by a diffusion-limited solid state reaction model (parabolic rate law). The corresponding half-lives of the AgNP ranged from minutes to hours. The increase of the sulfidation rate with increasing HA concentration may be explained by the adsorption of HA onto the AgNP surface facilitating the access of HS⁻ to the particle surface. The results from analytical transmission electron microscopy suggest that the AgNP were sulfidized asymmetrically in the absence of HA. In the presence of HA, the initially formed concentric core–shell Ag⁰–Ag₂S structures developed into hollow Ag₂S nanoparticles with increasing reaction time, possibly via the Kirkendall effect.