A novel heterogeneous catalyst was prepared by in situ immobilization of H5PMo10V2O40 (PMo10V2) on protonated g-C3N4 (pg-C3N4) under hydrothermal conditions. The results of potential, PL, FT-IR, XRD, and XPS indicated that PMo10V2 was immobilized on the protonated C-N=C of CN heterocycles. XPS and EPR confirmed the presence of V4+ in the catalyst due to electronic interaction between PMo10V2 and pg-C3N4. The textural and morphology properties of the catalyst were characterized by N-2 adsorption-desorption, SEM, and TEM. PMo10V2/pg-C3N4 showed excellent catalytic performance with 25.8% benzene conversion and 99.7% selectivity to phenol in the hydroxylation of benzene to phenol. The excellent catalytic performance of PMo10V2/pg-C3N4 could be attributed to high dispersion of PMo10V2 on pg-C3N4, the presence of V5+/V4+ redox pairs, and the cooperative benzene-activation capability of g-C3N4. The PMo10V2/pg-C3N4 catalyst can be easily recycled without loss of activity.