Bioactive molecules derived from seaweed extracts are revolutionary bio-stimulants used to enhance plant growth and increase yield production. This study evaluated the effectiveness of a commercially available seaweed liquid extract, namely, True-Algae-Max (TAM®), as a plant growth stimulant on nutritional, and antioxidant activity of Capsicum annuum. Three concentrations of TAM® (0.25, 0.5, and 1%) of various NPK: TAM® ratios were investigated via foliar spray, over 2017 and 2018 cultivation seasons, under greenhouse conditions. TAM® is rich in phytochemical compounds, such as ascorbic acid (1.66 mg g−1), phenolics (101.67 mg g−1), and flavonoids (2.60 mg g−1) that showed good antioxidant activity (54.52 mg g−1) and DPPH inhibition of 70.33%. Promoting measured parameter results stated the extensive potentiality of TAM® application, in comparison with conventional NPK treatment. Yield and composition of C. annuum were significantly improved in all TAM® treated groups, especially the TAM0.5% concentration, which resulted in maximum yield (4.23 Kg m−2) and significant amounts of profuse biological molecules like chlorophyll, ascorbic, phenolic compounds, flavonoids, and total nutrients. Compared to the NPK control treatments, C. annuum treated with TAM0.5% improved the total antioxidant activity of hot Pepper from 162.16 to 190.95 mg g−1. These findings indicate that the extract of seaweed can be used as an environmentally friendly, multi-functional biostimulant in the agricultural field for more sustainable production, in addition to reducing the use of hazardous synthetic fertilizers.