We present measurements of the time-dependent CP-violation parameters S and C in B0→η′K0 decays. The data sample corresponds to 384×106 BB̄ pairs produced by e+e- annihilation at the Υ(4S). The results are S=0.58±0.10±0.03 and C=-0.16±0.07±0. 03. We observe mixing-induced CP violation with a significance of 5.5 standard deviations in this b→s penguin dominated mode. © 2007 The American Physical Society.