Two field experiments were carried out at a private farm at Sakha Region, Kafr El-Sheikh Governorate, Egypt, during the winter seasons of 2020/2021 and 2021/2022. The experiment aimed to study the physiological response of onion plants to foliar spraying with methionine (50, 75 and 100 mg/L) and its nanoparticles (5, 10 and 15 mg/L). It is obvious from results that all applied treatments induced significant increases in most investigated parameters. The most significant and pronounced treatments were PEG- encapsulated methionine at 5mg/L followed by 100 mg/L methionine relative to control. It is worthy to mention that PEG- encapsulated methionine at 5mg/L significantly increased vegetative growth parameters (dry weight of leaves /plant by 144.62% and bulb fresh weight by 125.28 %); total soluble solids by 59.53%; total photosynthetic pigments by 210.81%;indole acetic acid by 54.34%;and yielded bulb fresh weight by 60.70 %. In addition, PEG- encapsulated methionine at 5 mg/L significantly increased total phenolic content by 1.18 times; flavonoid by1.26 times; free amino acid by 1.92 times; protein by 1.37 times; and total soluble sugars by 1.66 times relative to control. The least increases were recorded due to methionine at 50 mg/L and PEG- encapsulated methionine at 15 mg/L relative to control. It could be concluded that PEG- encapsulated methionine at low doses (5 mg/L) may be used as promising technique to increase onion productivity and quality.