<p>Chemoresistance remains a major barrier in treating oral squamous cell carcinoma (OSCC). This study investigated whether the marine-derived sesterterpenoid heteronemin (HET) suppresses chemoresistant OSCC (SAS-CR) cells and elucidated its underlying mechanisms. HET dose- and time-dependently reduced SAS-CR viability, inhibited clonogenic growth, and exhibited stronger cytotoxicity than cisplatin or 5-fluorouracil. Furthermore, HET induced S-phase arrest by downregulating proliferation and cell-cycle proteins (PCNA, c-Myc, cyclin A, cyclin D3, CDK4, and CDK6). Concurrently, it triggered intrinsic apoptosis, characterized by mitochondrial depolarization, upregulated cleaved caspase-3/PARP and Bax, and downregulated Bcl-2. This caspase-dependent apoptosis was partially reversed by pan-caspase inhibitor Z-VAD-FMK. Transcriptomic profiling linked these phenotypes to metabolic stress, revealing alterations in the tricarboxylic acid cycle, mitochondrial respiration, and copper homeostasis. Consistently, HET elevated intracellular Cu²⁺ levels and reduced FDX1, SDHB, and lipoylated DLST/DLAT protein expression-cytotoxic effects that were attenuated by the copper chelator tetrathiomolybdate (TTM). Additionally, HET increased reactive oxygen species (ROS) production, whereas ROS scavenger N-acetylcysteine (NAC) attenuated HET-induced apoptosis and restored cuproptosis-related markers. In a zebrafish model, HET demonstrated negligible toxicity while reducing tumor-associated fluorescence and FDX1 expression. Collectively, HET effectively suppresses chemoresistant OSCC through coordinated ROS-dependent apoptosis and cuproptosis-associated mitochondrial stress, supporting its development as a therapeutic candidate for refractory OSCC.</p>