ExoplanetSat combines the low cost CubeSat platform with an innovative two-stage attitude control system to detect exoplanets. ExoplanetSat will be capable of detecting transiting Earth-sized planets in the habitable zone of the brightest sun-like stars with a detection threshold of 7σ. The choice of targeting the brightest sun-like stars is motivated by the desire to conduct spectral follow-up observations to determine the habitability of exoplanet candidates. We present the design of the first three-unit ExoplanetSat, which will launch in the next two years under NASA's CubeSat Launch Initiative.