Seawater, the most abundant water resource on Earth, presents a potential solution for sustainable agriculture. However, its high salinity challenges plant growth and soil health, necessitating innovative approaches. This study evaluates the feasibility of using seawater as a foliar spray under freshwater irrigation for pepper plants. Treatments involved seawater from the Mediterranean and Red Seas, both as normal and algae-based nanoparticle forms, combined with a salt solution (vanadium, zirconium, titanium). Seawater from Gamasa and Safaga, applied at 1% and 5% concentrations, was tested in its normal and nanoparticle forms, with and without the salt solution through a completely randomized experimental design. Results indicated that normal seawater from Gamasa and Safaga, even at higher concentrations, showed limited improvement over the control. Algae-based nanoparticles demonstrated enhanced plant growth and productivity, particularly from Safaga. The combination of nanoparticle seawater with the salt solution significantly outperformed other treatments; with Safaga's combination, showing the highest increase in growth performance, photosynthetic pigments, fruit yield, and quality traits of pepper plants. This study highlights the potential of seawater for agricultural use, suggesting that algae-based nanoparticle treatment, especially when combined with a salt solution, can effectively enhance crop productivity. Future research should focus on optimizing these techniques to maximize benefits and minimize adverse effects, paving the way for seawater as a viable resource in sustainable agriculture.