The research aimed to evaluate the use of synthetic or natural auxins on the growth of tomato seedlings. The seeds of Solanum lycopersicum "Marmande" were sown in polystyrene plug plant trays (104 cells). Two doses of natural or synthetic exogenous auxins (200 ppm and 100 ppm) were supplied to the substrate through the irrigation water with an ebb and flow system 3, 11, and 17 days after sowing (05, 11, and 13^th BBCH growth stage, respectively). A commercial biostimulant based on Ecklonia maxima extracts (Basfoliar® Kelp SL Compo) was used as a source of natural auxin while 1-naphthaleneacetic acid NAA was used as a synthetic auxin. Seedlings supplied only with water were used as a control. The treatments had significant effects on many morphological and physiological parameters (plant height, stem diameter, plant fresh and dry weight, leaf number and area, stomatal conductance, plant water use, and water use efficiency). Seedlings treated with both doses of exogenous auxin provided via Ecklonia maxima extracts increased their fresh and dry weight by 31% and 37% respectively and were taller and leafier than the control seedlings. The use of NAA had a negative effect on plant height and stem fresh and dry weight but did not alter the other morpho-physiological parameters as compared to the control seedlings. The treatments with auxins from algae extract during nursery growth improved the performance of tomato seedlings but the benefits could be probably ascribed not only to auxins themselves but to the synergic effect of the other organic compounds contained in the product (amino acids, proteins, carbohydrates, and vitamins).