Internal concentration polarization (ICP) severely limits water flux performance in forward osmosis (FO). We investigated the use of ultrasound to mitigate ICP. Various parameters affecting the performance of the novel ultrasonically-assisted FO were studied, such as ultrasonic frequency and constant versus pulsed operation. With either deionized water or polyphenolic tannin solution as the feed and sodium sulphate as the draw solution, the water flux was nearly doubled for a thin film composite FO membrane upon the application of a 20 kHz ultrasound, with stronger enhancement achieved when the ultrasound was applied to the support layer of the membrane. High frequencies of 573 and 1136 kHz had much weaker effects. Pulsed application of ultrasound can significantly reduce the energy consumption of sonication. For the first time, the current study provides compelling evidence that ultrasonic vibrations applied to porous support structure of an FO membrane is highly effective in mitigating ICP.