A mouthpiece for use with a tooth whitening device which projects light towards a person's teeth and activates a whitening medium applied on the teeth. The mouthpiece has a teeth cover with a translucent part for allowing the light to pass through onto the whitening medium. The mouthpiece prevents saliva to leak from the person's mouth on the whitening device by having an upper and optionally a lower ridge covering the person's lips. The mouthpiece further comprises a fixture and protrusions for directing, stabilizing and securing the mouthpiece in the right position in the person's mouth.
