An illumination system and a projection device are provided. The illumination system includes an excitation light source, a wavelength conversion module and a light combining prism. The excitation light source emits an exciting light beam. The light combining prism has a first, second, and third surfaces. The exciting light beam enters the light combining prism through the first surface, and is totally reflected by the second surface to leave the light combining prism through the third surface and is transmitted to the wavelength conversion module. The exciting light beam is converted into a wavelength converted light beam by a wavelength conversion region of the wavelength conversion module or reflected by a reflecting region of the wavelength conversion module at different timings, and the wavelength converted light beam or the exciting light beam sequentially penetrates through the third surface and the second surface to leave the light combining prism.