To provide an optical transmission substrate, an optical transmission module and an optical circuit substrate capable of suppressing optical coupling loss of an optical element and a core member.

The optical transmission substrate includes a substrate 2 having a mounting region on the upper surface side thereon where the optical element 6 is mounted and a through hole penetrating in the vertical direction in the mounting region, a clad member 3 having an optical waveguide hole positioned in the through hole and penetrating in the vertical direction, the core member 4 positioned in the optical waveguide hole and having a higher refractive index than that of the clad member 3, and a light guide member 5 which is arranged from the upper side end surface 4A of the core member 4 to at least a part of the upper side end surface 3A of the clad member 3 and the refractive index thereof is the same or higher than that of the core member 4.