[Object] To provide an optical waveguide sheet including a mirror structure that can be fabricated at low cost and has a high smoothness, an optical transmission module, and a manufacturing method for an optical waveguide sheet. [Solving Means] An optical waveguide sheet according to the present technology includes a core, a cladding, and a mirror structure. The core extends in a first direction parallel to a first plane. The cladding is provided around the core. The mirror structure has a concave shape which is formed from the cladding to the core, the mirror structure including a first inclined surface that is parallel to a second direction perpendicular to the first direction and parallel to the first plane, is inclined with respect to the first plane, and includes a core region in which the core is exposed, and a bottom surface parallel to the first plane.