We use a novel electrostatic ion storage ring to measure the radiative lifetime of the upper level in the 3p⁵ ²P⁰_1/2 → 3p⁵ ²P⁰_3/2 spontaneous radiative decay in ³²S^- to be 503±54 sec. This is by orders of magnitude the longest lifetime ever measured in a negatively charged ion. Cryogenic cooling of the storage ring gives a residual-gas pressure of a few times 10^-14 mbar at 13 K and storage of 10 keV sulfur anions for more than an hour. Our experimental results differ by 1.3σ from the only available theoretical prediction [P. Andersson et al., Phys. Rev. A 73, 032705 (2006)].