Background To evaluate the usefulness of fresh onion (Allium cepa L.) leaves, a food waste, as a new food resource, we characterized the taste characteristics, volatile compounds, sensory properties, and antioxidant activity of fresh onion leaves compared with those of Welsh onion (Allium fistulosum L.) (green leaf, small variety) leaves. Results The total sugar (g/100 g), total organic acid (mg/100 g), and total free amino acid (mg/100 g) concentrations of fresh onion leaves were 2.12 ± 0.15, 730.02 ± 17.43, and 93.72 ± 7.17, respectively, and were significantly (p < 0.05) lower than those of Welsh onion leaves (2.38 ± 0.07, 907.23 ± 20.79, and 131.34 ± 10.22, respectively). In fresh onion leaves, dipropyl disulfide concentration was higher than five times that in Welsh onion leaves. Regarding the sensory properties of the boiled samples, the fresh onion leaves’ color was less favorable than Welsh onion leaves (p < 0.05), and the taste and aroma were indifferent. The total oxygen radical absorbance capacity value (μmol Trolox equivalents/100 g) and 1,1-diphenyl-2-picrylhydrazyl free radical-scavenging activity (μmol Trolox equivalents/100 g) of fresh onion leaves were 805.78 ± 100.32 and 406.70 ± 63.64, respectively, and differed insignificantly compared with Welsh onion leaves (888.00 ± 112.61 and 382.98 ± 26.08, respectively). However, the total phenolic content (mg gallic acid equivalents/100 g) and quercetin concentration (mg/100 g) of fresh onion leaves were 36.53 ± 2.53 and 5.71 ± 0.11, respectively, and were significantly (p < 0.05) higher than those of Welsh onion leaves (25.07 ± 2.02 and 0.18 ± 0.02, respectively). Additionally, total vitamin C (23.36 ± 0.62 mg/100 g) and β-carotene (1529.32 ± 167.77 μg/100 g) concentrations in fresh onion leaves were similar to those rich vegetables. Conclusions This is the first study to report fresh onion leaves’ sensory properties and volatile compounds. Additionally, fresh onion leaves could be a new food resource, presenting an alternative to Welsh onion leaves and acting as an excellent dietary source of antioxidants, including quercetin, vitamin C, and β-carotene.