A magnetic susceptor in a printable filament form is developed for the induction welding of thermoplastic composites. The susceptor is based on Ni particles embedded in a poly-ether-imide matrix. It is extruded and spooled to form a filament which can then be 3D-printed. The susceptor produces heat by hysteresis losses due to the magnetic properties of the Ni particles. As opposed to other typical electrically conductive heating elements, no percolation threshold needs to be achieved to produce heat as the Ni particles individually heat up when exposed to the induction coil’s magnetic field. The heating efficiency of the susceptor filament and its deposition by the fused filament fabrication technique are demonstrated. The susceptor is used to assemble all thermoplastic composite sandwich panels. The sandwich samples are tested by the flatwise tensile test and a tensile strength of 4.6 MPa is obtained, which is equivalent to or higher than reported strengths for typical aerospace-grade sandwich panels. The printable susceptor opens the way to new induction welding or heating applications as it can be printed on a surface to produce a desired heating pattern.