  We describe the purification of xenon from traces of the radioactive noble
gas radon using a cryogenic distillation column. The distillation column is
integrated into the gas purification loop of the XENON100 detector for online
radon removal. This enabled us to significantly reduce the constant $^{222}$Rn
background originating from radon emanation. After inserting an auxiliary
$^{222}$Rn 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
$^{222}$Rn activity concentration inside the XENON100 detector.
