An airbag for a vehicle occupant restraint system is provided. The airbag comprising a first and a second chamber, which can be inflated for the protection of a vehicle occupant; having an outflow element with an outflow duct via which the gas can flow out of the first chamber when the internal pressure in the first chamber exceeds the internal pressure in the second chamber and which can be compressed under the action of the internal pressure in the second chamber such that an outflow of gas out of the second chamber is counteracted, when the internal pressure in the first chamber is lower than the internal pressure in the second chamber, the outflow element has a two-layer material piece and the outflow duct is formed by folding in a portion of the material piece between the material layers of the material piece.