The longitudinal piezoelectric coupling factor for the width-extensional mode of a thin KNbO3 plate, k11′, has theoretically been predicted to be as high as 82.4% for the 43.5° rotated Z cut about the Y axis. To experimentally confirm the predicted value we fabricated a width-extensional vibrator using a 45° rotated Z cut plate about the Y axis, which is close to the maximum k11′ cut. However, the measured value of k11′ was 71.3%, appreciably lower than the predicted value. It is clarified that the lower coupling factor can be ascribed to the tilt of the electric-field vector from the major face of the plate, which results from an off-diagonal component of the dielectric tensor. To prevent the electric-field tilt, a polar multidomain structure consisting of two kinds of 90° domains is introduced into the width-extensional vibrator instead of a single-domain structure. It is demonstrated by finite element method simulations that the coupling factor approaches the predicted value (82.4%) as the number of domains formed in the vibrator increases. The measured coupling factor of a width-extensional vibrator with a polar multidomain structure is as high as 79.4%, which is close to the predicted value. An additional advantage of using this domain structure is its lower spurious response.