The adverse effects of chemical fertilizers on human health and environmental sustainability have driven a shift toward organic farming and sustainable soil amendments. Marine macroalgae, particularly Ulva lactuca, have gained attention as promising bioresources due to their rich biochemical composition and nutrient availability. This study explores the potential of U. lactuca-derived biochar and raw seaweed as soil amendments to enhance plant growth and physiological performance. Biochar was produced through slow pyrolysis at 450 ± 5°C under oxygen-limited conditions, yielding a material with low carbon (C) and nitrogen (N) content but elevated pH, phosphorus (P), and potassium (K) levels, similar to poultry manure biochar. A greenhouse experiment conducted in October 2023 evaluated the effects of raw U. lactuca at two application rates (SW1, SW2) and its derived biochar (BC1, BC2), both independently and in combination (BC1+SW1, BC1+SW2), on Pisum sativum L. seedling development. Growth parameters—including root depth, shoot length, fresh and dry biomass—and photosynthetic pigment concentrations (chlorophyll a, chlorophyll b, and carotenoids) were assessed 10 days after sowing. All treatments significantly improved seedling growth and pigment content compared to controls, with the highest biochar rate (BC2) and its combination with SW2 yielding the most pronounced effects. These findings highlight U. lactuca-derived biochar as a valuable organic soil amendment, particularly at higher application rates, with potential benefits for sustainable agriculture and crop productivity.