Brazil imports a large amount of onion and the producing states are restricted to the Central-South regions; therefore, it is essential to identify new areas with production potential. In the state of Tocantins, located in northern Brazil, there is no record of commercial onion cultivation for bulb production; consequently, it is important to determine the best date for the establishment of this crop. The aim of this research was to determine the adaptation and yield potential of onion planted on three dates in the town of Gurupi, Tocantins. Three experiments were established using the hybrids NUN 1205, Dulciana, and Cimarron, all of early-maturity, and three transplant dates: May 26, June 26 and July 26, 2015. A randomized complete block experimental design with four replicates was used. The bulb was evaluated in terms of commercial yield, average weight, shape and diameter, as well as plant height, pseudostem diameter and number of leaves. In postharvest, concentration levels of pyruvic acid, pH, size and bulb classification were determined. In July and June, commercial bulb yield was 27.90 and 58.05 t∙ha-1, respectively. The average bulb weight varied from 93 to 193 g∙bulb-1, where the round shape predominated. The most suitable month for transplanting onion in the region is June, which was when the highest yield was recorded. The classification of the bulbs obtained on the three dates and from the evaluated hybrids covers the demands of the market. In addition, the commercial yield was similar to that obtained in other producing regions. In July, the hybrid Cimarron obtained a lower yield than the national average; for this reason, its production is not recommended in that month.