This study evaluated the antibacterial and antifouling potential of ethanol–water extracts from the brown macroalga Ericaria amentacea collected from the northwestern Algerian coast, prepared using maceration and ultrasonication. The extracts exhibited notable bioactivity, showing significant growth inhibition against both bacterial and diomatic biofouling models. Antibacterial assays revealed the strongest activity against Escherichia coli and Listeria innocua, with inhibition zones of 20 ± 0.82 mm and 16.33 ± 0.46 mm, respectively, at 100 µg/mL. Moderate inhibition was observed against Staphylococcus aureus (11 ± 0.10 mm), Listeria ivanovii (9.33 ± 0.94 mm), and Candida albicans (8.67 ± 1.24 mm). In antifouling assays, all tested concentrations significantly suppressed the growth of the diatom Nitzschia sp. compared to the control, though the reference biocide Diuron demonstrated a stronger, irreversible effect. The half-maximal effective concentration (EC₅₀) for the algal extracts was 10.52 µg/mL, confirming their potent inhibitory capability, whereas Diuron exhibited a significantly lower EC₅₀ (≤ 5 µg/mL). These findings highlight E. amentacea as a promising source of natural bioactive compounds for developing environmentally friendly antibacterial and antifouling agents as alternatives to synthetic biocides.