A backlight unit includes a substrate, a plurality of light source assemblies disposed on the substrate, each of the light source assemblies including first to k-th light sources, where the k is a natural number greater than or equal to two, a plurality of sensing lines disposed on the substrate, the sensing lines respectively connected to the first light sources of the light source assemblies, a connection line disposed on the substrate, the connection line connecting the first to k-th light sources of each of the light source assemblies in series, and a controller connected to the sensing lines, and a width of the connection line is greater than a width of each of the sensing lines.