The invention is a capsule for containing a substance for preparing a beverage by placing the capsule in the cavity of a piston. The capsule has a body with a lateral wall, a rim and a sealing ring which is attached to the lower wall of the rim. The rim is connected to the lateral wall by a rounded transition. The sealing ring is one piece and has an upper face bearing on the lower wall of the ring and has a free lower face. The sealing ring has an inner periphery which is arranged to bear against the round transition of the lateral wall.