We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column was integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant ²²²Rn background originating from radon emanation. After inserting an auxiliary ²²²Rn emanation source in the gas loop, we determined a radon reduction factor of R>27 (95% C.L.) for the distillation column by monitoring the ²²²Rn activity concentration inside the XENON100 detector.