We report on THz time-domain spectroscopy measurements of flux grownKTiOPO4in the frequency range 0.6–7.0 THz (20 to 240 cm−1). Our results compare well with earlierRaman and far-infrared measurements. We theorize that the differences between the opticalphonon mode resonances reported in the literature are due to what growth parameters and meltcompositions are used to grow the crystals. We also report on a new feature at 1.4 THz (47 cm−1) © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement. Under a Creative Commons CC-BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/).