We have measured the 1 S13-2 S13 interval in positronium (Ps) to be 1 233 607 185 ± 15 MHz, in agreement to within 1% of the α3R QED prediction. The quoted 12-ppb uncertainty has equal contributions from the measurement of the Ps resonance relative to a Te2 absorption line and the calibration of the Te2 line relative to the deuterium 2S12-4P32 Balmer line.