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<i>Halocynthia roretzi</i>, the edible ascidian, has been demonstrated to be an important source of bioactive natural metabolites. Here, we reported a novel terpenoid compound named Halorotetin A that was isolated from tunic ethanol extract of <i>H. roretzi</i> by silica gel column chromatography, preparative layer chromatography (PLC), and semipreparative-HPLC. ¹H and ¹³C NMRs, ¹H-¹H COSY, HSQC, HMBC, NOESY, and HRESIMS profiles revealed that Halorotetin A was a novel terpenoid compound with antitumor potentials. We therefore treated the culture cells with Halorotetin A and found that it significantly inhibited the proliferation of a series of tumor cells by exerting cytotoxicity, especially for the liver carcinoma cell line (HepG-2 cells). Further studies revealed that Halorotetin A affected the expression of several genes associated with the development of hepatocellular carcinoma (HCC), including oncogenes (<i>c-myc</i> and <i>c-met</i>) and HCC suppressor genes (<i>TP53</i> and <i>KEAP1</i>). In addition, we compared the cytotoxicities of Halorotetin A and doxorubicin on HepG-2 cells. To our surprise, the cytotoxicities of Halorotetin A and doxorubicin on HepG-2 cells were similar at the same concentration and Halorotetin A did not significantly reduce the viability of the normal cells. Thus, our study identified a novel compound that significantly inhibited the proliferation of tumor cells, which provided the basis for the discovery of leading compounds for antitumor drugs.