Sulfated glycoproteins extracted for the first time from the Moroccan green seaweed Codium decorticatum were investigated for their ability to induce a natural defense metabolism in the roots and the upper leaves of tomato seedlings. The crude (AGB) and the purified fractions (AGP) were characterized chemically (Colorimetric assays) and structurally (SEC-MALS, GC-EI/MS, ATR-FTIR). The elicitor aqueous solutions (1 g/L) were applied by foliar spray and syringe infiltration into the internodal middle of 45-day-old tomato seedlings. Phenylalanine ammonia-lyase (PAL) activity, polyphenols, and lignin contents were measured in the roots and the leaves after 0 h, 12, 24, 48, and 72 h of treatment. The AGB and AGP extracts contained 37.67% and 48.38% of the total carbohydrates, respectively, and were mainly composed of galactose, glucose, arabinose, and a minor amount of xylose and rhamnose. They were characterized by an important molecular weight (Mw) > of 2000 × 103 g·mol−1 and a high degree of sulfation and protein (12–23% (w/w)), indicating that the extracted polysaccharides could be an arabinogalactan-rich protein present in the cell wall of the green seaweed C. decorticatum. Both crude and purified fractions exhibited an elicitor effect by inducing the PAL activity, the accumulation of phenolic compounds and lignin contents in the roots and the leaves of tomato seedlings. These responses were systemic in both the methods used (injection and foliar spray) and were mobilized throughout tissues that are not directly treated (roots and/or leaves). Regarding the elicitor activities, AGB and AGP presented globally similar patterns, which revealed the importance of crude extracts in the stimulation of plant immunity. These results suggest the new application of sulfated glycoprotein isolated from green seaweed in agriculture as inducers of natural defenses of plants