A directional backlight unit and a three-dimensional (3D) image display device including the directional backlight unit are provided. The directional backlight unit includes: a light guide plate, a light source configured to irradiate an incident surface of the light guide plate with a plurality of color lights, and a grating that includes a sub-grating configured to react to all of the plurality of color lights. The directional backlight unit may further include a color filter that corresponds to a plurality of color lights emitted from each sub-grating.