Marine organisms exhibit a rich chemical content that possesses unique structural features as compared to terrestrial metabolites. Among marine resources, marine algae are a rich source of chemically diverse compounds with the possibility of their potential use as a novel class of antimicrobial agents. The antimicrobial activities of Cystoseira schiffneri extracts were tested toward Staphylococcus aureus, Enterococcus faecalis, Escherichia coli and Pseudomonas aeruginosa. The ethyl acetate extracts for both fresh and dried Cystoseira schiffneri exhibited the highest activities with MICs values ranged from 0.078 to 0.729 mg/mL. The antifungal activities were evaluated against Candida albicans, Candida krusei, Candida parapsilosis and Candida glabrata. Ethyl acetate and dichloromethane extracts from fresh Cystoseira schiffneri recorded inhibitory activities higher than the positive control “fluconazole”.