An illumination system, including a light source, a reflective optical element, a polarization conversion element, a beam splitting element, and a homogenizing element, is provided. The light source provides a first light beam having a first polarization state and a second light beam having a second polarization state. The reflective optical element is disposed on a transmission path of the first light beam and the second light beam. The polarization conversion element is disposed between the light source and the reflective optical element. The beam splitting element is disposed between the light source and the polarization conversion element. The beam splitting element includes at least one first region and at least one second region. The homogenizing element is disposed on a transmission path of a light beam reflected by the beam splitting element. A projection device having the illumination system is also provided.