A thin backlight module includes a light guide plate, a light source module, and an adhesive layer. The light guide plate has a light-entering end and a bottom, wherein a plurality of troughs spaced at intervals are formed on a portion of the bottom connected to the light-entering end. The light source module is disposed corresponding to the light-entering end and includes a substrate and a plurality of light sources. The light sources are disposed on the substrate and distributed along the light-entering end, wherein each of the light sources corresponds to an interval between the adjacent troughs. The adhesive layer is attached to the substrate, faces the bottom of the light guide plate, and extends into the troughs to be attached to the light guide plate.