Studies estimate that nearly 2 million new cases of gastric cancer will occur worldwide during the next two decades, which will increase mortality associated with cancer and the demand for new treatments. Marine algae of theLaurenciagenus have secondary metabolites known for their cytotoxic action, such as terpenes and acetogenins. The speciesLaurencia obtusahas demonstrated cytotoxicity against many types of tumors in previous analyses. In this study, we determined the structure of terpenes, acetogenins, and one fatty acid ofLaurenciausing mass spectrometry (ESI-FT-ICR/MS). In vitro cytotoxicity assays were performed with adenocarcinoma gastric cells (AGS) to select the most cytotoxic fraction of the crude extract ofL. obtusa. The Hex:AcOEt fraction was the most cytotoxic, with IC509.23 µg/mL. The selectivity index of 15.56 shows that the Hex:AcOEt fraction is selective to cancer cells. Compounds obtained fromL. obtusawere tested by the analysis of crystallographic complexes. Molecular docking calculations on the active site of the HIF-2α protein showed the highest affinity for sesquiterpene chermesiterpenoid B, identified from HEX:AcOEt fraction, reaching a score of 65.9. The results indicate thatL. obtusapresents potential compounds to be used in the treatment of neoplasms, such as gastric adenocarcinoma.