An improved rubber stopper for a waterproof connector is very simple in structure, capable of ensuring that the terminal holding chamber of a connector housing is hermetically sealed and is prevented from coming out. A rubber stopper 10 for a waterproof connector is equipped with a columnar body portion 11 and a sealing protrusion 12 capable of adhering to the inside wall surface of a terminal receiving hole of a free cavity. The sealing protrusion 12 is equipped with an annular portion 13 which is protruded along the outer peripheral face of the body portion 11, and a thick-wall portion 14 which is thicker than the annular portion 13 in the axial direction. Each protrusion of the thick-wall portion 14 is equal in height to the annular portion 13 and has a substantially hemispherical shape smoothly continuous to the side face of the annular portion 13. The thick-wall portions 14 are protruded at equal intervals along the outer peripheral face of the annular portion 13.