A planar light source including a light guide plate (LGP) and a light source device is provided. The LGP includes a plate portion and at least one prism portion. The plate portion has a light incident surface, a reflective surface, a bottom surface, and a light exiting surface. The prism portion is disposed on the bottom surface of the plate portion, and has a first plane and a second plane. An included angle between the first plane and the bottom surface is smaller than an included angle between the second plane and the bottom surface, and a shortest distance between the first plane and the light incident surface is smaller than a shortest distance between the second plane and the light incident surface. The light source device is disposed adjacent to the light incident surface. The disclosure provides planar light source with good light uniformity.