In the coming years, the application of biostimulants in agriculture will become a fundamental tool for reducing chemical fertilization, increasing the efficiency of soils and crops to face up to climate change conditions, as well as for the recovery of soils. Following this context, we have evaluated the effect of garden waste compost tea (CT) in a ratio of 1: 5 (v/v) with water on root morphology of tomato var. Tres Cantos plants. The studied CT showed a relevant content in K2O, N, humic acids and, to a lesser extent, amino acids. Three treatments were proposed: water, optimal tomato Hoagland solution and compost tea, which were axenically prepared. The tomato seeds were sterilized, germinated and then transferred to the considered treatments contained in Petri dishes, where plants were grown for seven days. Then, root growth parameters were measured and it was observed that CT induces the growth of the main root, as well as the number of lateral roots. Moreover, indolacetic acid, indol-3-butyric acid, zeatine, 6-benzyladenine and gibberellic acid were measured, but no changes were observed between treatments. Thus, other direct or indirect pathways seem to be involved in CT-mediated tomato root modulation.