A protective cover for the mouthpiece of brass instruments is formed by three portions. The first cover portion is in the form of a molded tube or cylinder. The tube is sized to snugly fit over the cup of the mouthpiece and a part of the instrument shaft adjacent the cup of the mouthpiece. The second cover portion is a cup formed from a second tube or cylinder having one closed end. The cup is sized to be snugly mounted over the rim of the mouthpiece and an adjacent part of the first cover portion covering the cup of the mouthpiece. The first and second portions are molded from an elastomeric thermal insulating foam material and covered with a stretchable nylon shell. The third portion is in the form of an elastic band connected at one end to the first portion and at a second end to the second portion. The elastic band acts as a tether for the second portion when the second portion is removed for playing the brass instrument.