Improving qualitative and quantitative traits of the onion (Allium cepa L.) plant, a strategic crop, is essential to meet increasing agricultural demands and enhance crop quality. The current study, conducted over two successive growing seasons (2022/23 and 2023/24), aimed to investigate the synergistic effects of elemental sulfur, various potassium supplements and methionine amino acid on the growth, yield and quality of onion plants. The experiment was arranged in a split-split plot design, as the main factor was the application of elemental sulfur (applied at a rate of 360 kg ha-1 or non-application). The sub-main factor involved different forms of potassium supplements applied as foliar sprays at rate of 750 mg K2O L-1 [Control (no potassium), potassium silicate (K2SiO3), potassium citrate (K3C6H5O7), potassium nitrate(KNO3)], while the sub-sub main factor included methionine amino acid, either sprayed at 100 mg L-1 or not sprayed. The results show that the application of elemental sulfur significantly improved onion performance regarding growth, quantitative and qualitative traits (e.g., fresh and dry weights, leaf area, chlorophyll a & b contents, bulb weight and diameter, marketable yield, total dissolved solids, dry matter, vitamin C and anthocyanin pigment) across measured parameters. Potassium silicate emerged as the most effective potassium supplement, followed by potassium citrate then potassium nitrate, and lately the control. Additionally, methionine application enhanced both the growth, yield and quality traits of onions. The combined application of elemental sulfur, potassium silicate, and methionine produced the highest values for growth, yield and quality attributes. Finally, it can be concluded that the usage of elemental sulfur, potassium supplements, particularly potassium silicate, and methionine is so beneficial to enhance the performance of onions. This integrated approach not only maximizes crop yield but also improves the quality of onions, offering a sustainable pathway to augment onion production. Future research should explore the long-term effects of these treatments and their interactions with other agronomic practices to further optimize onion cultivation.