Rattling phenomena have been observed in materials characterized by a large cage structure but not in a simple ABO3-type perovskite because the size mismatch, if it exists, can be relieved by octahedral rotations. Here, we demonstrate that a stoichiometric perovskite oxide NaWO3, prepared under high pressure, exhibits anharmonic phonon modes associated with low-energy rattling vibrations, leading to suppressed thermal conductivity. The structural analysis and the comparison with the ideal perovskite KWO3 without rattling behavior reveal that the presence of two crystallographic Na1 (2 a) and Na2 (6 b) sites in NaWO3 (space group Im3̅) accompanied by three in-phase WO6 octahedral (a+a+a+) rotations generates an open space Δ ∼ 0.5 Å for the latter site, which is comparable with those of well-known cage compounds of clathrates and filled skutterudites. The observed rattling in NaWO3 is distinct from a quadruple perovskite AA'3B4O12 (A, A': transition metals) where the A (2 a) site with lower multiplicity is the rattler. The present finding offers a general guide to induce rattling of atoms in pristine ABO3 perovskites.