Objectives: To investigate the antihypertensive activity of aqueous extracts obtained from Malaysian coastal seaweeds and to determine the pharmacological mechanisms of the extracts on rat aorta in vitro. Materials and Methods: The antihypertensive activity of 11 species of seaweeds (5 brown, 1 red and 5 green algae) were tested by cumulative addition of the extracts to phenylephrine (PE)-precontracted Wistar-Kyoto (WKY) aortic rings in in vitro isometric contraction studies. Mechanisms for vasorelaxant effect were investigated in the presence of various antagonists. Results: Of the 11 species tested, 2 showed a vasorelaxant effect. Further investigation of the mechanisms of action of the aqueous extract of green alga, Cladophora patentiramea (AECP),showed that the vascular relaxant effect was endothelium- and concentration-dependent. A maximum relaxation of 45.8 ± 4.6% (n = 8, p < 0.001) was obtained at 0.1 mg/ml of extract, after which the response was found to reduce in a concentration-dependent manner to 15.7 ± 4.9% (n = 8, p < 0.001) at the highest extract concentration tested. Pretreatment of endothelium-intact aortic rings with Nω-nitro-L-arginine methyl ester (L-NAME, 30 µM), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 µM) and methylene blue (100 µM) resulted in a complete blockade of AECP-induced vasorelaxation. However, the relaxant effects of the extract were not blocked by atropine (1 µM), indomethacin (10 µM) and glibenclamide (10 µM), although the maximum relaxant responses were enhanced in the presence of glibenclamide. Conclusion: Our data showed that the in vitro vascular relaxant effect of AECPwas mediated through endothelium-dependent nitric oxide-cGMP pathway, and was not associated with the release of vasodilator prostaglandins, activation of muscarinic receptors, or ATP-sensitive potassium channels opening.