A rubber stopper is a small, tapered plug used to seal the openings of test tubes, flasks and other laboratory glassware. Stoppers made of cork are also available for this purpose. However, rubber stoppers are preferable for applications that require a tighter seal or a greater degree of chemical resistance.

The primary purpose of a rubber stopper is to prevent a gas or liquid from escaping its container during a scientific experiment. Rubber stoppers can also prevent the contamination of samples by protecting the contents of laboratory glassware from air. Last, the use of rubber stoppers allows experimenters to shake or mix solutions without spilling them.

The standard stopper is cylindrical in shape, with a tapered bottom end. Some rubber stoppers contain one or two holes to allow the insertion of pipettes, tubing or test equipment (e.g., a thermometer).

Rubber stoppers typically range in diameter from size 000 (0.5 inches) to size 16 (5 inches) at the top. The tapered bottom end ranges from five-sixteenths to 3.5 inches. Smaller stoppers are appropriate for test tubes and similar glassware; larger stoppers are appropriate for flasks and beakers.

The main properties that make rubber suitable for use as a laboratory stopper are its elasticity, chemical resistance and impermeability. Its elasticity allows it to form a tight seal against the inside of the glassware. Its chemical resistance makes it safe for use with many corrosive and otherwise reactive compounds. Its impermeability helps to prevent the escape of liquids and gases from the container.