To provide an optical sheet reducing interference fringes of light even when a rear surface-covering layer such as a sticking preventing layer is provided and to provide a backlight unit preventing uneven brightness by using the optical sheet.

The optical sheet 1 is provided with a transparent base material layer 2, an optical functional layer 3 formed on the front surface side of the base material layer 2 and the rear surface-covering layer 4 composed of a transparent resin laminated on the rear surface side of the base material layer 2 where the thickness of the rear surface-covering layer 4 is ≥4.5 μm and ≤6.4 μm. An acrylic resin is preferably used as the transparent resin constituting the rear surface-covering layer 4. The rear surface-covering layer 4 is formed by coating with a resin composition containing the transparent resin. The coating amount (expressed in terms of solid matter) is preferably ≥5.6 g/m2 and ≤9.5 g/m2.