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Rosemary (Rosmarinus officinalis L.) is an evergreen and a xerophytic shrub that is frequently employed in culinary, decorative, and industrial applications. It shows significant variations in biomass and essential oil (EO) yield due to effects of biotic and abiotic factors. Biostimulants are widely used in agriculture, and in organic agricultural systems, they may contribute significantly to the stability and/or to the increase in crop yields, ensuring respect for the environment. The aim of this study was to assess the effect of different types of biostimulants on the yield performance of rosemary. Four commercial formulations of biostimulants based on Eklonia maxima, Ascophyllum nodosum, fulvic acids, and protein hydrolysates were used for the tests. Water was used as a control. Six applications were performed. At harvest, several morphological and productive parameters were determined. All parameters were affected by biostimulant application. The highest fresh and dry yields were obtained with the application of fulvic acids (13.1 t ha−1 of fresh biomass and 4.3 t ha−1 of dry biomass) and protein hydrolysates (13.6 t ha−1 of fresh biomass and 4.4 t ha−1 of dry biomass) with values 60% higher than that of the control. Regarding the EO content in the dry biomass, the highest value (1.72% v/w) was found in control plants, whilst the lowest (1.14%) was observed in plants treated with fulvic acids. In the case of the EO yield, the lowest value (46.5 kg ha−1) per unit area was found in the control. The application of Eklonia maxima and protein hydrolysates produced the highest EO yield values, 65 kg ha−1 and 66.5 kg ha−1, respectively. This study highlights the use of biostimulants to increase the yield performance of rosemary when grown in an organic agricultural system.