A rubber bung is a rubber stopper used to seal laboratory flasks or other apparatus during experimentation.

The rubber bung may be used to seal a flask because the user may require the contents to be mixed via shaking the flask or may require that the contents be kept inside the flask and prevented from leaking out.

The rubber bung may also include one or more holes so as a glass tube or Laboratory Funnel may be inserted through the bung and into another piece of apparatus.

In all cases the bung keeps the experimentation environment completely sealed so as liquids or gases cannot escape.