We report type I phase‐matched blue second‐harmonic generation from periodically segmented channel ion‐exchanged waveguides in KTiOPO4with output wavelengths from 0.38 to 0.48 &mgr;m and efficiencies exceeding 50%/W cm2. Evidence is presented suggesting that these efficiencies result from ferroelectric domain reversals induced by waveguide fabrication. Waveguide structure, wavelength, and output power characteristics are presented.