A new Ultrasound-assisted Extraction (UAE) of phenolic compounds has been optimized and validated. Central Composite Design (CCD) in conjunction with Response Surface Methodology (RSM) was applied based on six factors and three levels to evaluate the significances of studied factors prior to optimize the UAE conditions. The effects of solvent composition (25−75% methanol in water), extraction temperature (10−50oC), solvent-solid ratio (2.5:1−5:1), solvent pH (2−7), amplitude (30−70%) and cycle (0.2−0.7 s-1) on the extraction yields were taken into account. Subsequently, extraction kinetic (5−30 min) was studied to confirm the full recovery of phenolic compounds extracted from the matrix. The levels of phenolics in the UAE extracts were quantified by Ultrahigh Performance Liquid Chromatography using a Photo-diode Array detection (UPLC-PDA) in less than 4 min of analysis run time. A complete validation of the method was performed presenting recoveries between 72 to 111% with high precisions (in average, CV was lower than 5% for both repeatability and intermediate precision). The validated method was successfully applied to the analysis of phenolics content in four corn cultivars including black, yellow, red and white. The proposed method was found to be reliable and is suited for the rapid extraction of phenolics from corn grain.