Acne vulgaris is often associated with acne-related bacteria such as Cutibacterium acnes. In this study, we investigated the antibacterial effects of the methanol extract of the algae Ecklonia cava and its solvent-soluble extract against C. acnes. Among five solvent fractions, the ethyl acetate (EtOAc) fraction exhibited the strongest antibacterial activity against C. acnes. Furthermore, the EtOAc-soluble extract exhibited the highest total phenolic contents among the five solvent fractions tested. The EtOAc subfraction 07 (Fr. 07) extract showed the highest antibacterial effect against C. acnes and isolated C. acnes along with minimum inhibitory concentration (MIC) ranging from 32 to 64 μg/mL. Additionally, MICs of antibiotics against antibiotic-resistant C. acnes strains were substantially reduced when antibiotics were combined with Fr. 07, suggesting that Fr. 07 restore the antibacterial activity of the antibiotics. The fractional inhibitory concentration indices clearly revealed an additive synergistic effect of Fr. 07 with antibiotics. The results of the present study suggest a potential role for E. cava in the control of infections related to acne vulgaris.