A novel technique is proposed and experimentally demonstrated to achieve a quasiphase matching (QPM) condition for second‐harmonic generation (SHG) in a nonlinear optical waveguide with a periodically poled LiNbO3waveguide. The fundamental wave satisfying the QPM conditions is automatically generated when the laser diode (LD) and the optical waveguide are optically connected. Using the proton‐exchanged LiNbO3waveguide with a periodically poled grating, which was fabricated by Ti diffusion and the InP/InGaAsP LD with antireflection‐coated facets, it was confirmed that the LD oscillated at the wavelength satisfying the QPM condition, and SHG was observed simultaneously. Using a Ti:sapphire laser, it was also confirmed that the SHG was realized at the third order of QPM condition, as was expected.