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Seaweed, in particular, brown seaweed, has gained research interest in the past few years due to its distinctive phenolic profile that has a multitude of bioactive properties. In order to obtain the maximum extraction efficiency of brown seaweed phenolic compounds, Response Surface Methodology was utilized to optimize the ultrasound-assisted extraction (UAE) conditions such as the amplitude, time, solvent:solid ratio, and NaOH concentration. Under optimal conditions, UAE had a higher extraction efficiency of free and bound phenolic compounds compared to conventional extraction (stirred 16 h at 4 °C). This led to higher antioxidant activity in the seaweed extract obtained under UAE conditions. The profiling of phenolic compounds using LC-ESI-QTOF-MS/MS identified a total of 25 phenolics with more phenolics extracted from the free phenolic extraction compared to the bound phenolic extracts. Among them, peonidin 3-<i>O</i>-diglucodise-5-<i>O</i>-glucoside and hesperidin 5,7-<i>O</i>-diglucuronide are unique compounds that were identified in <i>P. comosa</i>, <i>E. radiata</i> and <i>D. potatorum,</i> which are not reported in plants. Overall, our findings provided optimal phenolic extraction from brown seaweed for research into employing brown seaweed as a functional food.