Tungsten was joined to ferritic-martensitic steel (FMS) for application in a plasma facing component. Zirconium foil was investigated as an interlayer material for the joining of W to FMS. Repeated hot isostatic pressing (HIP) was conducted for the fabrication of W/FMS joints. The first HIP was performed at 950°C under 100 MPa for 1.5 h (diffusion joining stage), and the second HIP was executed at 750°C under 70 MPa for 2 h (tempering stage). The Zr interlayer formed a sound interface between W and FMS with no observable pores and cracks. The joining strength of W/FMS measured by a shear test was about 54 MPa. Elemental diffusion was observed along the hetero-interfaces of W/Zr and Zr/FMS. At the W/Zr interface, a thin layer of W–Zr inter-phase was observed. At the Zr/FMS interface, no intermetallic compound was formed, however, fine Zr grains featuring body-centered tetragonal lattice structures were formed near the interface.