Capsule for preparing a beverage

A capsule intended for containing a substance for preparing a beverage by placing the capsule in the cavity of a piston, the capsule comprising a body with a lateral wall bordered on either side by a bottom and by a rim surrounding an opening of the body, the rim having a lower wall projecting laterally from the lateral wall, the rim is connected to the lateral wall by a rounded transition, the capsule being fitted with a single one-piece sealing ring attached to the lower wall, the sealing ring having an upper face abutting the length of the lower wall and a free lower face, the sealing ring also having an inner periphery which is arranged bearing against the rounded transition of the lateral wall, the capsule having an inner bearing surface and an outer bearing surface each projecting from the rim in an axial direction forming therebetween a groove.