An illumination system includes a light source, a wavelength conversion element and a light guide column. The light source provides an excitation beam. The wavelength conversion element has a wavelength conversion portion for converting the excitation beam into a converted beam. The light guide column is disposed between the light source and the wavelength conversion element and located on a transmission path of the excitation beam. The light guide column has a first end, a second end opposite to the first end, a third end and a fourth end opposite to the third end. The third and fourth ends respectively face the light source and the wavelength conversion element. The third end receives the excitation beam. The excitation beam exits the first or fourth end, and the converted beam enters the light guide column through the fourth end and exits the first end. A projection device is also provided.