The Southwest region of Pará state, Brazil, imports 100% of all onion consumed from other regions of the country, which generates market dependence and raises the product's costs. The present study investigated parameters related to germination, agronomic performance and postharvest characteristics of five onion cultivars at different planting densities. The experiment was conducted in two stages. In the first, in a greenhouse, the germination kinetics was evaluated. In the second, in the field, the seedlings were transplanted into beds. The experimental design was in complete randomized blocks in a 5 x 3 factorial scheme with three replications. The treatments consisted of the combination of five cultivars (Mata Hari, Dulciana, Cimarron, NUN 1205-F1 and Vale Ouro IPA 11) and three line spacing (20, 30 and 40 cm) 8 cm between plants. As for the germination rate (GR), the results allowed to highlight the cvs. Cimarron and Vale Ouro IPA 11 with an 80.55% and 77.77% of GR, respectively. There was a significant difference for total bulb productivity, highlighting Vale Ouro IPA 11 in the 8 x 20 cm spacing. For the bulb diameter, cv. Vale Ouro IPA 11 obtained the best results in all classes. In the average weight of the bulbs, cv. Vale Ouro IPA 11 in the 8 x 40 cm spacing was the best treatment with 60.77g. As for total soluble solids, cv. Vale Ouro IPA 11 presented °Brix higher than the others, being significant at 1%. These results demonstrate that it is possible to produce quality onions in the Southwest region of Pará, which would reduce prices, since all commercialized onions come from regions outside the state.