The invention relates to an odor seal for a vacuum toilet drain system whose cross section in the state of no throughflow tapers off in such a way in the downstream direction that the interior walls on the downstream end of the odor seal rest against each other so that the odor seal in a downstream direction allows throughflow while in the opposite direction it blocks throughflow, wherein the downstream end of the odor seal is closed in the state of throughflow and no throughflow of the odor seal. Furthermore, the invention relates to a drainage device comprising such an odor seal and a sieve which is arranged upstream of the odor seal. Moreover, the invention relates to a vacuum toilet system for an aircraft, comprising such an odor seal or a drainage device.