Abstract: According to the present invention, an optical waveguide includes a core layer having a first surface and a second surface having a front and back relationship with each other, the core layer including a core portion extending along a core axis and a side clad portion, a first cover layer provided on the first surface, the first cover layer having an adhesive surface on an opposite side of the core layer, and a second cover layer provided on the second surface, the second cover layer having an opposite surface on an opposite side of the core layer. The optical waveguide has a sheet shape and has a first recess portion that is open to the adhesive surface. When the adhesive surface is viewed in plan view, the first recess portion includes a first groove extending along a first axis that intersects with the core axis. The optical waveguide is used by being adhered to an adhesion target via an adhesive layer in contact with the adhesive surface.