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The removal of toxic Cr(VI) is a hot topic in the environmental remediation field. In this work, a Ti-based metal–organic framework (MOF) (MIL-125(Ti)-NH2) was successfully functionalized by introducing amidoxime groups for the first time. The functionalized material (MIL-125(Ti)-AO) exhibited excellent Cr(VI) adsorption performance with a maximum adsorption capacity of 271 mg·g–1according to Langmuir fitting. More importantly, during the adsorption process, Cr(VI) could be simultaneously reduced to less toxic Cr(III) species, and the residual concentration of chromium in the treated water was below the drinking water limit (0.05 mg·L–1) recommended by WHO. The effects of initial pH, contact time, and the initial concentration of Cr(VI) and the presence of competitive ions on the Cr(VI) adsorption performance of MIL-125(Ti)-AO were systematically investigated. The excellent Cr(VI) removal performance of MIL-125(Ti)-AO was attributed to the synergistic effects of simultaneous adsorption of Cr(VI) and Cr(III) by a [Ti–O] bond when Cr(VI) was reduced to Cr(III) by amidoxime groups.