A backlight unit includes a light source which supplies light; a light guide which transmits the light; an optical sheet on the light guide, and including a guide member extending from a side surface of the optical sheet; and a reflection sheet under the light guide. The reflection sheet reflects the light toward the optical sheet and includes a protrusion guide member which extends from a side surface of the reflection sheet and toward the light guide. The guide member of the optical sheet is coupled to the protrusion guide member of the reflection sheet, and restricts movement of the optical sheet.