Soilless cultivation, as one of the modern agricultural methods, can create a balance between world production and consumption per capita and respond to the market's need for food, including onion greens, in a timely manner. The aim of this study was to investigate the possibility of producing onion bulbs under hydroponic conditions. This experiment was conducted in a factorial form and in a randomized complete block design with four planting densities {1, 2, 6, 12} plants per cell and two onion cultivars Ramehramzi and Primavera in three replications in the field of the Horticultural Sciences Department of Shahid Chamran University of Ahvaz during the fall of 2023. The results showed that the main effect of cultivar, density and the interaction effect of cultivar on density on bulb diameter, neck diameter and their ratio to each other, and bulb fresh and dry weight were significant. The highest and lowest bulb diameters were observed in the Primavera cultivar at a density of 1 plant per cell (41.8 mm) and at a density of 12 plants per cell (15.1 mm), respectively. The Primavera cultivar at a density of 1 plant per cell (1.31 g) and the Ramehramzi cultivar at a density of 12 plants per cell (1.9 g) had the highest and lowest fresh bulb weight, respectively. The average production per square meter for the Primavera cultivar at a density of 12 plants per cell was 25411.4 g and for the Ramehramzi cultivar at a density of 12 plants per cell was 17882.35 g. Also, the results of the correlation coefficients between the traits studied in this study showed that there was a significant correlation between the traits studied. According to the results obtained, it can be said that the Primavera cultivar at a density of 1 plant per cell had the highest onion bulb diameter and weight in the hydroponic culture medium.