Abstract: Pentavalent uranium ((U(Ⅴ)) has historically been overlooked in environmental geochemistry due to its inherent instability and susceptibility to oxidation and disproportionation under ambient conditions. However, emerging studies have increasingly indicated that U(Ⅴ) can persist under specific environmental conditions, particularly within the structural framework of iron oxides, and may play a significant role in uranium mobility, transformation, and long-term environmental fate in natural systems. The geochemical behavior of U(Ⅴ) was systematically investigated through a Fe²⁺-induced phase transformation of ferrihydrite to goethite, focusing on the incorporation mechanism, isotopic fractionation and geochemical stability of U(Ⅴ) within the resulting goethite structure. The results demonstrate that during the transformation of ferrihydrite to goethite, the uranyl ions ( \mathrmU\mathrmO_2^2+ ) can structurally substitute for Fe(Ⅲ) in the octahedral lattice sites of goethite. X-ray absorption near-edge structure spectroscopy at the U M₄-edge using high-energy resolution fluorescence detection (HERFD-XANES) confirms that U(Ⅴ) is the dominant oxidation state of uranium incorporated into the goethite lattice, and it is structurally stabilized in the form of uranate species. Notably, no characteristic peaks corresponding to uranyl moieties are observed, supporting the absence of labile U(Ⅵ) surface complexes. Isotopic analysis using multi-collector ICP-MS further reveals a clear and systematic isotopic fractionation pattern during uranium redox transformations, following the order of δ(²³⁸U_U(Ⅵ))＜δ(²³⁸U_U(Ⅴ))＜δ(²³⁸U_U(Ⅳ)). This fractionation trend is consistent with nuclear field shift theory, reflecting the preferential enrichment of heavier isotopes in more reduced uranium species and providing new insights into redox-driven uranium isotope behavior in natural systems. Furthermore, long-term environmental stability experiments indicate that lattice-incorporated U(Ⅴ) in goethite remains chemically stable even under strongly reducing or oxidizing conditions, with no detectable changes in oxidation state or mineral phase. The above results suggest that the incorporation of U(Ⅴ) into iron oxide minerals may represent a previously unrecognized uranium sequestration pathway in geological and environmental settings. Overall, this work contributes to a broader understanding of the role of iron oxides in uranium geochemistry and offers new perspectives on the environmental fate of uranium in subsurface. The results have significant implications for uranium ore genesis, isotope-based environmental reconstruction, and the development of mineral-based strategies for uranium contamination remediation and nuclear waste immobilization.