Iron phosphate (FePO₄) with moderate working voltage is an appealing cathode material for room temperature rechargeable sodium-ion batteries (SIBs), while the low electronic conductivity and poor cycle performance remarkably impede its possible energy storage applications. In this work, we report a simple route to fabricate graphene oxide (GO) hybridized amorphous FePO₄·2H₂O composite for SIBs. By directly mixing FeCl₃·6H₂O and GO (mass controlled by x wt% of the mass of FeCl₃·6H₂O) in an agate mortar with pestle, Fe³⁺ ions could be bound by the oxygen-containing functionalities on the surface of GO nanosheets. After adding NH₄H₂PO₄ powder to the above mixture, a direct room temperature solid state reaction between the GO-confined Fe³⁺ ions and PO₄³⁻ polyanions results in an in-situ hybridization of amorphous FePO₄·2H₂O and GO, i.e. the target electroactive material (denoted as FePO₄@GO-x) is attained. In-depth mechanism for the solid state reaction is demonstrated by FTIR and XPS techniques. The optimized hybrid of amorphous FePO₄·2H₂O and GO (FePO₄@GO-15 composite) exhibits the best sodium storage performance. A high reversible charge capacity of 129.0 mAh g⁻¹ is delivered at a current density of 100 mA g⁻¹ for the first cycle, which is around 90% of the theoretical capacity (144 mAh g⁻¹) of FePO₄·2H₂O. The composite also exhibits outstanding rate performance, superior reversibility, and satisfactory long-lasting cycle stability. When cycled at a high current density (800 mA g⁻¹ at 31 °C), the composite cathode still delivers a charge capacity of 51 mAh g⁻¹ over 1000 cycles, corresponding to 67% of the first charge capacity. The results presented clearly indicate that the hybrid of amorphous FePO₄·2H₂O and GO is a promising cathode material for SIBs.