The present investigation aimed to study antimicrobial activities in marine microalgae, screened from Moroccan coastlines. Ethanolic extracts were prepared from the microalgae and evaluated each against the bacteria: Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus, the yeast Candida albicans and the fungus Aspergillus niger. The highest antibacterial activity was found in the extract of Tetraselmis sp. which exhibited an inhibitory effect against the three bacteria with a MIC of 2.6 to 3.0 mg extract per mL culture. Extracts from the other microalgae: Dunaliella Salina, Nannochloropsis gaditana, Dunaliella sp., Phaeodactylum tricornutum and Isochrisis sp. each showed inhibitory activity against E. coli or P. aeruginosa with a MIC of 2.6 to 4.3 mg extract per mL. The extract of N. gaditana also inhibited S. aureus growth. However, the extracts from the microalgae, Chaetoceros sp. and Chlorella sp. showed no effect under the applied experimental conditions. All the tested extracts inhibited the growth of C. albicans; the highest activity was obtained from N. gaditana with a MIC of 4.0 mg extract per mL culture. Aspergillus niger appeared to be resistant to the effect of the extracts. The observed antimicrobial activities were linked to the contents of the extracts in fatty acids, carotenoids and phenolic compounds. In conclusion, the studied microalgae could be considered as a potential natural source of bioactive compounds with antimicrobial activities.