The reliable joining of plasma-facing tungsten (W) to structural reduced activation ferritic/martensitic steel is a key challenge in manufacturing the first wall for future fusion reactor blankets. The large mismatch in their coefficients of thermal expansion leads to significant residual stress, and brittle intermetallic compounds readily form at the interface, both of which severely degrade joint performance. Introducing a copper (Cu) interlayer between W and steel is an effective strategy to mitigate stress concentration and suppress brittle phases. However, achieving their high-strength metallurgical bonding is difficult due to the immiscibility of W/Cu and the limited solubility of Cu/steel. In this study, a robust W/Cu/steel joint was successfully fabricated by brazing steel to a high-performance W/Cu slice using a Cu–Ge filler metal. The results show that a body-centered cubic-structured interdiffusion layer, approximately 10 nm thick, forms at the W/Cu interface, with its interplanar spacing about clear larger than that of pure W. At the Cu/steel interface, a discontinuous network-like metallurgical structure is established. All joints fractured within the Cu interlayer regardless of the holding time, demonstrating high interfacial bonding strength. A maximum shear strength of 240 MPa was achieved, accompanied by good ductility, indicating an excellent strength-ductility synergy. This work reveals the atomic-scale interdiffusion mechanism at the W/Cu interface and the bonding behavior at the brazed Cu/steel interface. It provides an efficient route for fabricating first-wall components and offers key insights into the interfacial heat transfer and load-transfer behavior in W/Cu or W/Cu/steel plasma-facing components under fusion-relevant conditions.