Abstract: In this study, a porous porphyrin-based metal–organic framework/reduced graphene oxide ((Fe–P)_n–MOF/graphene) composite was prepared via hydrothermal reduction with tetracarboxyphenyl porphyrin (TCPP) and iron(III) chloride (FeCl₃) as the main raw materials. The composite was designed to serve as a selective adsorbent for yttrium ions (Y³⁺). The adsorption performance of the composite toward Y³⁺ was investigated. The results indicated that the maximum adsorption capacity of the composite was 102.1 mg/g. The adsorption process followed the quasi-second-order kinetic and Langmuir isotherm models, indicating a monolayer chemical adsorption mechanism. The composite material was comprehensively characterized to analyze its adsorption mechanism. Using density functional theory (DFT) calculations, the electrostatic potential distribution in (Fe–P)_n–MOF and the binding energies of its adsorption sites toward metal ions were simulated to further determine the Y³⁺ adsorption mechanism of (Fe–P)_n–MOF. The composite demonstrated excellent selective adsorption of Y³⁺ from rare-earth leaching solutions and maintained a recovery rate exceeding 90% even after more than five regeneration cycles. Thus, (Fe–P)_n–MOF/graphene is a promising Y³⁺ adsorbent.