Rare earth elements (REEs), as powerful geochemical tracers, are effective tools for distinguishing sedimentary and hydrothermal origins in surface environments. In this study, the origin of soils and river sediments in the Kooh-Sorkh area (northeast Iran), which formed in sedimentary rocks but have been affected by argillic hydrothermal alteration, has been investigated. A total of 30 samples, including 26 soil samples and 4 river sediment samples, were analyzed using ICP MS. The REE data were normalized to Post Archean Australian Shale (PAAS), chondrite and Primitive Mantle (PM). Key geochemical indices, including (La/Yb)ₙ, (La/Sm)ₙ, Eu/Eu*, Ce/Ce*, and (Gd/Yb)ₙ, were calculated. Additionally, high field strength elements (HFSE) such as Zr, Ti, and Nb were used to determine the origin. All three normalization methods show enrichment of light rare earth elements (LREE), a positive Eu anomaly, a slightly negative Ce anomaly, and a negative Gd anomaly. HFSE ratios indicate a limited contribution from sedimentary sources to pedogenesis and confirm the influence of an external hydrothermal origin. The La, Th, Sc ternary diagram place most samples out of the sedimentary origin field but closed to basic magma sources, which clearly distinguishes them from materials of pure sedimentary origin. Overall, the REE patterns provide strong evidence of hydrothermal processes related to epithermal systems in the region.