ABSTRACT The correct use of hydrogels in some situations can increase seedling survival rates, plant growth, and the water-retention capacity of the growing medium. A technique for the production of a nanocomposite hydrogel using calcium montmorillonite (NC-MMt) was recently developed. However, additional research is necessary in order to validate this innovative input, particularly for the production of vegetable crops seedlings. In this context, the main objective of the present study was to evaluate the growth and development of seedlings, tomato hybrid ‘BRS Nagai’, in substrates containing different amendments of hydrogel NC-MMt. The trial was conducted in a 3x3 factorial arranged in a complete randomized blocks design, with three replications. Three substrates (peat moss, pine bark, and coconut peat) corresponded to the first factor and three rates of the NC-MMt hydrogel (0%= R1, 1.5%= R2 and 2.0%= R3, on a w/w basis) corresponded to the second factor. Each plot was composed of 16 plants that were assessed after three weeks. Overall, peat moss-based and pine bark-based substrates resulted in higher values for most of the analyzed traits: plant emergency percentage, plant height, stem diameter, leaf area, plant height/shoot dry weight ratio, root dry weight, shoot dry weight/root dry weight ratio and the Dickinson Quality Index. Rates of NC-MMt hydrogel displayed significant responses only to root superficial area and root volume. NC-MMt hydrogel amendment (mainly 1.5%) combined with specific substrates (mainly peat moss-based substrate) was able to improve the growth of ‘BRS Nagai’ tomato seedlings, with no observed toxic effects.