Background: Seaweed polysaccharides are highly active natural substances having valuable applications. The present study was conducted to characterize the physico-chemical properties of sulphated polysaccharides from three Mediterranean brown seaweeds (Cystoseira sedoides,Cystoseira compressaandCystoseira crinita) and to evaluate their anti-radical, anti-inflammatory and gastroprotective activities.Methods: The different rates of neutral sugars, uronic acids, L-fucose and sulphate content were determined by colorimetric techniques. The different macromolecular characteristics of isolated fucoidans were identified by size exclusion chromatography equipped with a triple detection: multiangle light scattering, viscometer and differential refractive index detectors, (SEC/MALS/VD/DRI). Anti-inflammatory activity was evaluated, using the carrageenan-induced rat paw edema test in comparison to the references drugs Acetylsalicylate of Lysine and Diclofenac. The gastroprotective activity was determined using HCl/EtOH induced gastric ulcers in rats and to examine the antioxidant effect of fucoidans in the three species, the free radical scavenging activity was determined using 1,1-diphenyl-2-picrylhydrazyl.Results: The pharmacological evaluation of the isolated fucoidans for their anti-inflammatory, and their gastroprotective effect established that these products fromC. sedoides,C. compressaandC. crinitaexhibited a significant anti-inflammatory activity at a dose of 50 mg/kg, i.p; the percentages of inhibition of the oedema were 51%, 57% and 58% respectively. And, at the same dose, these fucoidans fromC. sedoidesandC. compressashowed a significant decrease of the intensity of gastric mucosal damages compared to a control group by 68%, whereas, the fucoidan fromC. crinitaproduced a less gastroprotective effect. Furthermore, the isolated fucoidans exhibited a radical scavenging activity.Conclusion: The comparative study of fucoidans isolated from three species of the genusCystoseirashowed that they have similar chemicals properties and relatives anti-radical, anti-inflammatory and gastroprotective activities which are found to be promising.