This study aimed to understand the effect of commercial seaweed extract as a biofertilizer, named True-Algae-Max (TAM®), on the yield, nutritional, antioxidant, and cytotoxic activity of Eruca vesicaria. Three concentrations of TAM® (5, 10, and 15%) were studied by foliar spray over the two cultivation years (2016 and 2017) without any chemical fertilizer, along with a control consisting of synthetic nitrogen, phosphorus and potassium (NPK) fertilizers. The yield and composition of E. vesicaria were significantly improved in all treatments, particularly at 10% concentration of TAM®, which resulted in maximum yield (1.99 kg m−2) and significant amounts of chlorophyll, carotenoids, phenolic compounds, flavonoids and total nutrients. Compared to the NPK control, E. vesicaria grown with 10% of TAM® improved total antioxidant activity from 41.80 to 49.36 mg g−1 and cytotoxicity from 25.30 to 60.40% with an IC50 value 85.7 µg mL−1 against the hepatocellular carcinoma cell line (HepG2). These findings indicate that seaweed extract can generally be used as a safe potential multifunctional biofertilizer in the agricultural field. The use of seaweed as a biofertilizer could potentially help mitigate the adverse effects of main nutrient deficiencies, diminishing the use of chemical fertilizers.