An optical waveguide sensor includes a substrate and an optical waveguide. The optical waveguide includes a core and a lateral clad. The core extends in a spiral shape above a surface of the substrate. The lateral clad is disposed in a same layer as the core above the surface of the substrate and is in contact with either side surfaces of the core. At least a part of a surface of the core located opposite from the substrate is a transmission surface from which light leaks and is absorbed by a detected object.