Objective: This study was carried out to detect presence of some secondary metabolites that may have antimicrobial activity and to evaluate the antioxidant activity and cytotoxicity of methanol extracts of six marine macroalgae belonging to green algae (Chlorophyceae), brown algae (Phaeophyceae) and the red algae (Rhodophyceae) collected from the intertidal area of the Sudanese Red Sea coast near Port Sudan. Methods: Preliminary phytochemical analysis, the DPPH radical scavenging and cytotoxicity screening were carried out in methanol extracts of the six macroalgae as per standard methods with few modifications. Results: Analysis revealed that secondary metabolites with higher medicinal activities such as saponin, cumarin, flavonoids, alkaloids, tannins triterpens and steroids were present. Anthraquinones, and cyanogenic glycosides were absent in all algal extracts. Most of samples were inactive in DPPH radical scavenging and has no cytotoxic effects. Conclusion: The study demonstrated clearly that macroalgae of the Red Sea can be significant sources of important compounds which may be used in formulation of drugs by the pharmaceutical industries.