Objective: To extract and identify the chemical composition of the polysaccharide isolated from the Eucalyptus cultivated in Lebanon and to evaluate its antioxidant activity. Materials: The water-soluble polysaccharide was isolated from Eucalyptus leaves, and its structure was identified by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, and carbon-13 nuclear magnetic resonance. The antioxidant activity of the active ingredient was screened for its radical scavenging ability using 2,2-diphenyl-1-picryl-hydrazyl radical (DPPH) test. Results: The results of the DPPH test have shown that fucoidan, the polysaccharide isolated from Eucalyptus, exhibited almost the same antioxidant activity against DPPH· as the ascorbic acid did at 100 μg/mL. Conclusions: This natural molecule extracted from a medicinal plant has a promising antioxidant activity and could be used in pharmaceutical and medical applications.