An optical semiconductor device of which the moisture resistance and the like are improved and the manufacturing method thereof are provided. An optical semiconductor device of the embodiment is configured to include an optical semiconductor element on a surface of which a circuit portion including a light-receiving or light-emitting element is formed; a terminal portion which is provided on a back of the optical semiconductor element and electrically connected with the circuit portion; a covering layer which covers the surface of the optical semiconductor element and is made of a transparent material; and sealing resin which covers side faces of the covering layer and of the optical semiconductor element. The circuit portion and the terminal portion may be connected by a rewiring pattern.