Partition member, planar light source, and liquid crystal display device

Provided are a partition member, a planar light source, and a liquid crystal display device, in which thermal shrinkage of the partition member is suppressed in the planar light source having the partition member. This planar light source is provided with: a substrate; a plurality of light sources disposed on the substrate; and at least one partition member disposed on the substrate, the partition member having: a plurality of first wall portions having first ridge lines extending in a first direction; a plurality of second wall sections each having a second ridge line extending in a second direction intersecting the first direction; and a plurality of partition regions that include two opposing first wall sections and two opposing second wall sections and that are surrounded by the first ridge line and the second ridge line in plan view, the partition regions being disposed in the first direction and the second direction. At least one first cutout portion is provided on at least one of the first ridge lines,