We report the results of resonant x-ray scattering experiments performed at the Np M4,5 edges in NpO2. Below T0=25 K, the development of long-range order of Np electric quadrupoles is revealed by the growth of superlattice Bragg peaks. The polarization and azimuthal dependence of the intensity of the resonant peaks are well reproduced assuming anisotropic tensor susceptibility scattering from a triple-q longitudinal antiferroquadrupolar structure. Electric-quadrupole order in NpO2 could be driven by the ordering at T0 of magnetic octupoles of Γ5 symmetry, splitting the Np ground state quartet and leading to a singlet ground state with zero dipole-magnetic moment.
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