Within this thesis, a mouthpiece was designed and developed as a part of a rebreather. To improve safety during the dive, the mouthpiece includes two main functions. In the first function, the mouthpiece is in the closed circuit mode, where the mouthpiece serves as the interface between the diver and the circuit of the diving apparatus. The second function is achieved by a mechanical switch to the open circuit mode, in which the circuit in the mouthpiece is closed and breathing gas is obtained directly from a gas storage tank. Based on requirements, which were developed as part of a market analysis, concepts and possible solutions fulfilling the requirements were developed. The most promising solution was advanced and optimized. The geometry was modeled and detailed using CAD. Several new developed components combined with available existing parts led to the mouthpiece as a system. Parallel to the design of the geometry a material selection was carried out and resulted in two appropriate materials (ASA and POM) for the application. Further, manufacturing processes have been selected for the production of the geometry, such as injection moulding, gluing and mechanical finishing. Using injection moulding simulation, the geometry could be optimized for a polymer-appropriate design. Through the production and testing of some series of prototypes during the design process, virtual results could be validated in reality. The conclusion of this work was an optimized geometry that fulfilled the requirements in the best way concerning function, design, materials and technology.