A field experiment was conducted in 2017-2018 in the Agricultural Experiment Station owned by the University of Warmia and Mazury in Olsztyn. The experiment had a randomized block design with three replications. The first experimental factor was common onion cultivar: ‘Whitney Wing F1’, ‘Spirit F1’ and ‘Red Baron’. The second experimental factor was the effect of two plant growth biostimulants, Calleaf Aminovital and Maximus Amino Protect, on the yield and quality of onions. In the control treatment, plots were sprayed with water. Total yield, marketable yield and the percentage of marketable yield in total yield, the weight, vertical diameter and horizontal diameter of each onion bulb, the content of dry matter, total sugars, reducing sugars, L-ascorbic acid and nitrates (V) in onions were determined in the study. The evaluated cultivars differed significantly in the weight and size of onions. Onions of cv. ‘Whitney Wing F1’ were characterized by higher yield, and higher weight, vertical diameter and horizontal diameter of bulbs than the remaining cultivars. The concentrations of dry matter, total sugars, L-ascorbic acid and nitrates (V) were higher in cv. ‘Red Baron’. The content of L-ascorbic acid and nitrates (V) was significantly affected by the applied biostimulants, and it was higher in onions treated with Maximus Amino Protect. The interaction between the experimental factors had a significant effect on the weight of a single onion bulb, and the concentrations of dry matter and L-ascorbic acid in the edible plant parts.