Since natural colorants have been used for dyeing for years, synthetic dyes have replaced them because they are more affordable and convenient to use. However, as synthetic dyes have been shown to contribute to pollution and have an adverse effect on the environment, scientists and researchers are once again focusing on environmentally friendly and sustainable colorants. Natural dyes are those that come from natural sources and have thus been shown to combat some of the current environmental problems including biodegradability and the reduction of water and air pollution, which not only helps the environment but also has been shown to be hygienic. Red onion peels may be used as a natural dye to produce a spectrum of warm, earthy colors. Because red onion peels contain natural pigments called anthocyanins, they are well-known for their potential as a natural dye. In this research, natural colors were extracted from red onion peel by drying, crushing, and boiling it in water. Thereafter, the extracted solution was applied on to polyester fabric using an ultrasonic technique in the presence alum as mordant (alum) by varying dye concentration, treatment time, temperature, and mordant concentration. The dyed samples were tested for color yield (K/S value) and color fastness to light, washing and rubbing. Samples dyed with ultrasonic technique have produced light to medium colors with good K/S value and fastness properties. Thus, this can be inferred that ultrasonic technique for natural dyeing of polyester is plausible with reasonable K/S values and fastness properties. It may be preferred to HT dyeing because it requires less water and energy compared to traditional dyeing method.