Terbuthylazine (TBA) is one of the most used herbicides in agriculture, forestry, and transportation sectors, particularly since 2007 when widely used atrazine has been banned. Soon after its pre-emergence application, TBA residues can be spread from soil to the entire aquatic environment. Biodegradation of TBA in the soil is much faster than its abiotic degradation in water, where it is proved to be relatively persistent and harmful to a range of aquatic organisms. Therefore, the efficiency of fly ash (FA)-treatment of watercourse samples fortified with TBA was tested in laboratory conditions. Deciduous hardwood-derived FA was produced in a cogeneration plant facility and subjected to in-depth characterization using SEM and XRD methods. The results showed that the addition of 1% (w/v) of the finest FA fraction, with a nano-to-micro-sized structure and alkaline Ca/Mg/Si-enriched mineralogy, can reduce the TBA half-life in the watercourse sample by more than 30-fold. The TBA dissipation in FA-amended water was well described with the first-order kinetics (R²>0.98). TBA and its environmentally relevant degradation product desethylterbuthylazine (DET) were not detected in the water sample 48 hours after the FA addition, whereas nearly 80% of TBA was found in the untreated sample. The XPS analysis of post-treated FA suggested a plausible involvement of O, Ca, K, Mg, and Si in TBA adsorption. A basis for the development of innovative FA-based polymers targeted to decontamination aquatic ecosystems loaded with TBA and other hazardous (agro)chemicals were proposed.