Microbial infections cause complicated health influences along with bad economic impacts. In the present investigation, three dominant seaweeds namely, Amphiroa anceps, Corallina officinalis and Sargassum filipendula were collected from different Egyptian sites at the Red Sea and Mediterranean Sea during autumn 2019. Organic extracts of the three algae were screened for their antibacterial activity against three pathogenic bacteria Salmonella typhiimurium, Staphylococcus aureus and Escherichia coli, in addition to in vitro antiviral activity against Rotavirus (RV), and Coxsackie virus B3 (CVB3) that cause severe diseases in human. Organic extract of A. anceps, C. officinalis and S. filipendula inhibit E. coli cells by 57.1%, 85.7%, and 91.4%, respectively. The highest level of concentration of the C. officinalis extract (100 µg/mL) inhibits 100% of Staphylococcus aureus cells followed by S. filipendula and A. anceps extract which inhibit 82.5% and 75% of S. aureus. Similarly, the highest concentration of C. officinalis extract inhibits S. typhiimurium by 80%. The extract of A. anceps exhibited a high antiviral effect against RV infection with TI = 22 and virus titers lessened by 2.75 log TCID50 followed by extractions of C. officinalis with TI = 18.3 and virus titers reduced by 2.5 log TCID50. Against CVB3 infection, the extract of A. anceps causes the highest antiviral activity with TI = 15 and reduce the viral titers by 2.5 log TCID50, followed by extractions of C. officinalis with TI = 8.8 and inhibition of virus titers by 1.75 log TCID50. Extract of S. filipendula displayed the lowest antiviral effects against RV and CVB3 infection with TI = 2.4 and 1.4, respectively. The obtained results clarified that the extract of three marine seaweeds maintains a potent antimicrobial activity, making them a future promising source of new antimicrobial drugs