Humic substances can increment plant development, promoting the growth of shoots and roots, but their use in the production of seedlings is little studied. The aim of this study was to evaluate the effect of different doses of humic substances in promoting the growth of onion seedlings of the Alfa São Francisco Ciclo VIII variety. The experimental design was completely randomized, with five replications and six treatments: control with application of water and 5 doses (5, 10, 15, 20 and 25 mL L-1) of humic substances containing 10% fulvic acid. The application was performed 28 days after sowing (DAS), by immersion of trays with the seedlings in the solution according to each treatment. The following characteristics were evaluated at 48 DAS (seedling transplanting time): shoot height, root length, pseudostem diameter, shoot fresh mass, shoot dry mass, root fresh mass, root dry mass, foliar area, foliar volume, root volume and root area. The use of humic substances influenced the evaluated characteristics. The greatedy growth promotion of onion seedlings occurred between the concentrations of 17 and 20 mL L-1 of humic substances in the immersion solution.