A one piece mouthpiece with a mandibular re-positioning extension for reducing snoring, and in particular, to a dental orthosis for easy engagement with the upper dentition of a person and intended to eliminate snoring. The mouthpiece is made from a deformable material with a shape complementary to a person's upper teeth and dental arch. The mouthpiece generally has an arched channel which embraces the upper teeth of a person. The mouthpiece includes an attached downward anterior mandibular extension located at the back of the top of the arch of the mouthpiece.