We have observed the magnetic resonant mode in the superconducting state of the ironbased superconductor K x Fe 2−y Se 2. It is centred at 12.7 ± 0.2 meV at Q = (0.25, 0.5, 0.5), using the unfolded Fe-sublattice scheme. By studying the temperature dependence of the resonance, we have shown that the mode is linked to the superconducting state and has an associated change in the low energy spectral weight at T c. Despite the resonance meeting a well defined nesting condition that indicates a superconductor with homogeneous phase, it appears the resonance does not disappear sharply at T c , unlike in the analogous iron arsenide superconductors.