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<i>Ulva</i> sp. is known to be a source of bioactive compounds such as ulvans, but to date, their biological activity on skin commensal and/or opportunistic pathogen bacteria has not been reported. In this study, the effects of poly- and oligosaccharide fractions produced by enzyme-assisted extraction and depolymerization were investigated, for the first time in vitro, on cutaneous bacteria: <i>Staphylococcus aureus</i>, <i>Staphylococcus epidermidis</i>, and <i>Cutibacterium acnes</i>. At 1000 μg/mL, poly- and oligosaccharide fractions did not affect the growth of the bacteria regarding their generation time. Polysaccharide <i>Ulva</i> sp. fractions at 1000 μg/mL did not alter the bacterial biofilm formation, while oligosaccharide fractions modified <i>S. epidermidis</i> and <i>C. acnes</i> biofilm structures. None of the fractions at 1000 μg/mL significantly modified the cytotoxic potential of <i>S. epidermidis</i> and <i>S. aureus</i> towards keratinocytes. However, poly- and oligosaccharide fractions at 1000 μg/mL induced a decrease in the inflammatory potential of both acneic and non-acneic <i>C. acnes</i> strains on keratinocytes of up to 39.8%; the strongest and most significant effect occurred when the bacteria were grown in the presence of polysaccharide fractions. Our research shows that poly- and oligosaccharide <i>Ulva</i> sp. fractions present notable biological activities on cutaneous bacteria, especially towards <i>C. acnes</i> acneic and non-acneic strains, which supports their potential use for dermo-cosmetic applications.