Resumo. The use of biofertilizers in agriculture has proven to be a promising strategy to optimize plant rooting, promoting a more vigorous and healthier root development. In this context, this study evaluated the effect of the biofertilizer derived from Kappaphycus alvarezii, cultivated in São Paulo (Kal-SP) and Santa Catarina (Kal-SC), on the rooting of basil plants (Ocimum basilicum L.). An entirely randomized design was used, with ten plants per treatment in a growth chamber. Four concentrations of the biofertilizer (1%, 3%, 5%, and 7%) were tested, using distilled water as a control. At the end of the experiment, morphological parameters (fresh and dry mass and root length) and biochemical parameters (total carbohydrates, total soluble sugars, and total starch) were evaluated. The root length showed significant increases with Kal-SC at concentrations of 3% (58%), 5% (55%), and 7% (54%) compared to the control. For fresh weight, the increases were 262%, 287%, and 312% at the same concentrations, and 200% for Kal-SP at 5%. In terms of dry weight, the increases were 257%, 277%, and 284% for Kal-SC and 204% for Kal-SP. Biochemically, higher amounts of total starch were observed in plants treated with Kal-SC at 3%, 5%, and 7%, as well as with Kal-SP at 1%, 3%, and 5%, with maximum increments of 82% (Kal-SP 3%). Thus, it is verified that the biofertilizer from K. alvarezii not only stimulates the rooting of basil but also alters the carbohydrate balance in the treated plants.