An optical semiconductor device includes: a package having a bottom portion and a sidewall portion; a semiconductor chip having an optical element formed on one surface thereof and having an opposite surface to the one surface fixed to the bottom portion of the package; a transparent member fixed to the semiconductor chip so as to cover the optical element; and a sealing resin filling a space between the package and the semiconductor chip. The sidewall portion has in an upper part thereof an overhang portion that projects toward inside of the package. The transparent member is exposed from a window portion formed by the overhang portion.