South Africa’s highly diverse marine biota includes several endemic marine red algae of the Laurencia genus. Cryptic species and morphological variability make the taxonomy of Laurencia plant challenging, and a record of the secondary metabolites isolated from South African Laurencia spp. can be used to assess their chemotaxonomic significance. In addition, the rapid development of resistance against antibiotics, coupled with the inherent ability of seaweeds to resist pathogenic infection, supported this first phycochemical investigation of Laurencia corymbosa J. Agardh. A new tricyclic keto-cuparane (7) and two new cuparanes (4, 5) were obtained alongside known acetogenins, halo-chamigranes, and additional cuparanes. These compounds were screened against Acinetobacter baumannii, Enterococcus faecalis, Escherichia coli, Staphylococcus aureus, and Candida albicans, with 4 exhibiting excellent activity against the Gram-negative A. baumanii (minimum inhibitory concentration (MIC) 1 μg/mL) strain.