In this study it is proposed the introduction of an expansion gas in high pressure water to maximize the cavitation caused by the application of ultrasound to improve the extraction of phenolic compounds from pomegranate peel. Different combinations of ultrasound power (US-Pwr), expansion gas initial pressure (N₂-Pi), system pressure (SP) and particle size of sample were evaluated using water as solvent. The use of US-Pwr and N₂-Pi individually or combined improved the extraction process proving higher yields. SP was an important parameter affecting extraction yield, showing an inverse relation between its increase and extraction yield. Although higher yields were produced with samples with smaller particles, the combination of ultrasound and expansion gas had a positive effect on the process independently of particle size, promoting an increase of 20–26% in yield. These results suggest an enormous potential to be explored with the introduction of an expansion gas in pressurized liquids in processes assisted by ultrasound for the extraction of phenolic compounds from natural products using green solvents.