In conventional superconductors, the interaction that pairs the electrons to form the superconducting state is mediated by lattice vibrations1 (phonons). In high-transition-temperature (high-Tc) copper oxides, it is generally believed that magnetic excitations might play a fundamental role in the superconducting mechanism because superconductivity occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds2. Indeed, a sharp magnetic excitation termed 'resonance' has been observed by neutron scattering in a number of hole-doped materials3, 4, 5, 6, 7, 8, 9, 10, 11. The resonance is intimately related to superconductivity12, and its interaction with charged quasi-particles observed by photoemission13, 14, optical conductivity15, and tunnelling16 suggests that it might play a part similar to that of phonons in conventional superconductors. The relevance of the resonance to high-Tc superconductivity, however, has been in doubt because so far it has been found only in hole-doped materials17. Here we report the discovery of the resonance in electron-doped superconducting Pr0.88LaCe0.12CuO4- (Tc = 24 K). We find that the resonance energy (Er) is proportional to Tc via Er 5.8kBTc for all high-Tc superconductors irrespective of electron- or hole-doping. Our results demonstrate that the resonance is a fundamental property of the superconducting copper oxides and therefore must be essential in the mechanism of superconductivity.