Provided are: an optical waveguide that relatively easily expands a spot size and that can suppress an increase in optical coupling loss with another optical waveguide element; and an optical component and variable-wavelength laser that use the optical waveguide. The optical waveguide is provided with: a cladding member; and a core layer that is disposed within the cladding member and that is formed as an elongated body having a rectangular cross-sectional shape from a material having a higher refractive index than the material configuring the cladding member. Here, the cross-sectional shape of the core layer is characterized in having a rectangular shape in which the length in the lateral direction is at least 10 times the length in the vertical direction.