A planar light source includes a light source, a light guide member, and a sectioning member. The light guide member has a light source placement part in which the light source is disposed and a section defining part having a groove surrounding the light source placement part. The sectioning member has light reflectivity and is disposed in the section defining part. A first portion in the section defining part that is furthest from a center of the light source in a plan view has a transmittance higher than a transmittance of a second portion in the section defining part that is closest from the center of the light source in the plan view.