Cladophora crispata, a diffusive green freshwater algum in Syria, forms a rich source of nutrients, especially sulfated polysaccharides (SPs). The SPs were extracted from Cladophora crispata by ultrasonic-assisted extraction using different types of solvents (methanol-70%, ethanol-70% and acetone-70%) at various temperatures (40, 60 and 80 °C) and for different extraction times (30, 75 and 120 min). Maximum yield of SPs (13%) and highest total soluble carbohydrate (74.36%) were achieved using acetone-70% as a solvent at 40 °C for 120 min. Purification of crude SPs was performed using Sephadex gel G-100. The purification process afforded two fractions (F1 and F2) for all treatments. The monosaccharides for the crude and for the F1 and F2 fractions were identified by thin-layer chromatography to consist of rhamnose, galactose, xylose and ribose. The chemical composition and antioxidant activity (DPPH and ATBS assays) of the crude and fractions were determined. FT-IR analysis revealed that the fractions consist of sugar units, protein units, sulfate groups and uronic acids, the content of which was determined following standard methods. Highest sulfate content was obtained upon extraction at low temperature (40 oC). The sulfate content had a significant strong positive correlation with the DPPH scavenging activity, but a weak negative one with the ABTS values. On the other hand, the uronic-acid content had a significant negative correlation with the DPPH values and a positive one with the ATBS values. Highest scavenging activity on DPPH radical (35.82%) was observed for the F2 fraction from the acetone-70% extract which had the highest sulfate content, while the highest scavenging effect on ABTS (39.71%) was found for the F1 fraction from the ethanol-70% extract which had the highest uronic-acid content.