Biostimulants are a better alternative to expensive organic fertilizers used in certified organic crop production. Seaweed extracts (Stimplex®), derived from brown algae (Ascophyllum nodosum) present in marine water, can be used as a "biostimulant" in agriculture to enhance plant yield and improve plant growth. The aim of this study was to elucidate the effect of Stimplex® on the yield of leafy green vegetables produced in an organic farming system. A Stimplex® greenhouse trial was conducted in fall 2015 and spring 2016 at the Tennessee State University organic research farm. Six leafy greens –lettuce, mustard, kale, Swiss chard, amaranths and collards– were foliar sprayed with Stimplex® (treated) bi-weekly for six weeks at the recommended dose of 3.2 ml/L (0.4 oz/gal of water), and six were untreated (control). Seedlings were raised in the greenhouse from organic seeds in pots, and organic management practices were followed per National Organic Program standards throughout the study. For each leafy green, a total of five treated potted plants and five untreated potted plants were used in three replications. Results showed that plants treated with Stimplex® were more vigorous, healthier and increased yield over untreated plants, except for amaranths. The percentage increase in plant yield of all crops was between 11% and 35%. The highest and lowest increase in plant yield was observed in the mustard greens (35.8 %) and Swiss chard (11.0%). Results suggest Stimplex® stimulates higher yields in leafy greens.