Diabetes mellitus is a prolonged health condition that needs persistent attention to prevent acute complications. Research has shown that millions of people are infected with diabetes globally which results in a lot of deaths worldwide. The need for an efficient and reliable expert system is necessary to ensure a quality diabetes diagnosis. This research entails the design and implementation of a portable and easy-to-use expert system. The waterfall model which is a progressive design process mostly used in software development processes involves the development process being able to flow steadily through the software development lifecycle. It was the most preferred for this work primarily because the model prescribes a systematic approach to software development which starts with a well-defined, understood specification of requirements and moves through to deployment of the system in a linear form. The software developed proved to be efficient and reliable for use since it gathers its knowledge from several medical specialists.