<p>The aim of this study was to evaluate the effect of gypsum amendments—flue gas desulfurization gypsum (FGDG) and phosphogypsum (PG)—on soil aggregate stability and susceptibility to water erosion in two contrasting soil types: arable soil from the Szreniawa River catchment (SRC) and permanent grassland soil from the Grajcarek Stream catchment (GSC). Soil samples were collected in accordance with the Polish Standard, and aggregate stability was determined using a wet sieving method. Additionally, electrical conductivity, pH, Ca²⁺ concentration, and total organic carbon (TOC) content were analysed. The results showed that the response to gypsum amendments differed significantly between the two soils. The arable soil from SRC, characterised by relatively low organic carbon content and high Ca²⁺ concentration, exhibited reduced aggregate stability after water treatment, and the applied amendments had a limited stabilising effect. In contrast, the grassland soil from GSC, with higher TOC content, showed greater initial stability and responded more positively to gypsum application, resulting in greater resistance to aggregate breakdown. Overall, the effectiveness of gypsum amendments in enhancing soil structure and reducing susceptibility to water erosion depends on soil properties, particularly organic matter content, particle size distribution, and land use. The obtained results highlight the importance of site-specific management when applying gypsum-based soil amendments.</p>