An optical semiconductor device includes a wiring board including a first surface and a second surface, a cover disposed to face the first surface, an optical semiconductor element disposed on the first surface, a plurality of electrodes disposed on the second surface, and a resist layer disposed on the second surface and located at least between the plurality of electrodes. A ventilation hole that penetrates the first surface and the second surface is formed in the wiring board. The second surface includes a disposition region in which the resist layer is disposed and a non-disposition region in which the resist layer is not disposed. The non-disposition region includes a first region in which the ventilation hole is disposed and a second region that reaches an edge of the second surface from the first region.